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Chapter 1: Introduction
Biogas typically refers to a (biofuel) gas produced by the
anaerobic digestion or fermentation of organic matter including
manure, sewage sludge, municipal solid waste, biodegradable waste
or any other biodegradable feedstock, under anaerobic conditions
[1].
Composition of Biogas varies based on the source used for
anaerobic digestion. Table 1: Composition of Biogas
Matter Raw Biogas Scrubbed Biogas
Methane, CH4 50-75% 95-98%
Carbon dioxide, CO2 25-50% 3-5%
Hydrogen sulphide, H2S 0-3%
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Chapter 2: Background Review
Chapter 2.1: Techniques of Scrubbing
There are several techniques being used for scrubbing biogas
depending upon the type of concentration level required and also
upon the limitation of temperature and pressure.
Scrubbing of CO2
A variety of processes are being used for removing CO2 from
natural gas in petrochemical industries. Several basic mechanisms
are involved to achieve selective separation of gas constituents.
These may include physical or chemical absorption, adsorption on a
solid surface, membrane separation, cryogenic separation and
chemical conversion.
Physical Absorption
For biogas scrubbing, physical/chemical absorption method is
generally applied, as it is effective even at low flow rates, at
which the biogas plants generally operating at. Also the method is
less complicated, requires lesser infrastructure and is cost
effective.
One of the easiest and cheapest methods involves the use of
pressurized water as an absorbent. The raw biogas is compressed and
fed into a packed bed column from the bottom; pressurized water is
sprayed from the top. The absorption process is, thus a
counter-current one. This dissolves CO2 as well as H2S in water,
which are then collected at the bottom of the tower. The water is
recycled to the first scrubbing tower. This perhaps is the simplest
method for scrubbing biogas.
After scrubbing, 5-10% of CO2 is left in biogas. This process is
cost effective as the compound being consumed is pressurized water
that is cheap and can be recycled easily. Currently this process is
in use in the sewage sludge based plants in Sweden, France and USA
[5].
Chemical Absorption
Chemical absorption involves the formation of reversible
chemical bonds between the solute and the solvent. Regeneration of
the solvent, therefore, involves breaking of these bonds and
correspondingly, a relatively high energy input. Chemical solvents
generally employ either aqueous solutions of amines, i.e. mono-,
di- or tri-ethanolamine or aqueous solution of alkaline salts, i.e.
sodium, potassium and calcium hydroxides.
Adsorption on Solid Surface
Adsorption involves the transfer of solute in the gas stream to
the surface of a solid material, where they concentrate mainly as a
result of physical or Vander wall forces. Commercial adsorbents are
generally granular solids with a large surface area per unit
volume. By a proper choice of adsorbent, the process can remove
CO2, H2S, moisture and other impurities either selectively or
simultaneously from biogas.
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Gas purification can also be carried out using some form of
silica, alumina, activated carbon or silicates, which are also
known as molecular sieves.
Adsorption is generally accomplished at high temperature and
pressure. It has a good moisture removal capacity, is simple in
design and easy to operate. But it is a costly process with high
pressure drops and high heat requirements.
Other Methods
Membrane Separation:
The principle is that some components of the raw gas could be
transported through a thin membrane (< 1 mm) while others are
retained. The transportation of each component is driven by the
difference in partial pressure over the membrane and is highly
dependent on the permeability of the component in the membrane
material. For high methane purity, permeability must be high. Solid
membrane constructed from acetatecellulose polymer has permeability
for CO2 and H2S up to 20 and 60 times, respectively, higher than
CH4. However, a pressure of 2540bar is required for the process.
This process has low reliability and high cost due to frequent
replacement of expensive membrane [5].
Cryogenic Separation
In a cryogenic method, crude biogas is compressed to
approximately 80bar. The compression is made in multiple stages
with inter-cooling. The compressed gas is dried to avoid freezing
during the cooling process. The biogas is cooled with chillers and
heat exchangers to -45 C, condensed CO2 is removed in a separator.
The CO2 is processed further to recover dissolved methane, which is
recycled to the gas inlet. By this process more than 97% pure
methane is obtained. No data is available on investment and
operational cost. This process involves high cost because it
requires generating sub-zero temperatures [5].
Chemical Conversion
To attain extremely high purity in the product gas, chemical
conversion method can be used. It reduces the undesirable gas
concentrations to trace levels. Usually the chemical conversion
process is used after bulk removal has been accomplished by other
methods. One such chemical conversion process is methanation, in
which CO2 and H2 are catalytically converted to methane and water.
Chemical conversion process is extremely expensive and is not
warranted in most biogas applications.
Due to highly exothermic nature of the methanation reactions,
the removal of the heat from the methanator is a major concern in
the process design. The requirement of the large amount of pure
hydrogen also makes the process generally unsuitable [5].
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Scrubbing of H2S H2S is always present in biogas, although
concentrations vary with the feedstock. It has to be removed in
order to avoid corrosion in compressors, gas storage tanks and
engines. H2S is poisonous and corrosive as well as environmentally
hazardous since it is converted to sulfur dioxide by combustion. It
also contaminates the upgrading process. H2S can be removed either
in the digester, from the crude biogas or in the upgrading
process
Dry Oxidation Process
It can be used for removal of H2S from gas streams by converting
it either into sulfur or oxides of sulfur. This process is used
where the sulfur content of gas is relatively low and high purities
are required. A small amount of oxygen (26%) is introduced in the
biogas system by using an air pump. As a result, sulfide in the
biogas is oxidized into sulfur and H2S concentration is lowered.
2H2S + O2 2S + 2H2O
This is a simple and low cost process. No special chemicals or
equipments are required. Depending on the temperature, the reaction
time and place where the air is added, the H2S concentration can be
reduced by 95% to less than 50ppm. However, care should be taken to
avoid overdosing of air, as biogas in air is explosive in the range
of 612%, depending on the methane content [5].
Adsorption on Iron Oxide
H2S reacts with iron hydro-oxides or oxides to form iron
sulfide. The biogas is passed through iron oxide pellets, to remove
H2S. When the pellets are completely covered with sulfur, these are
removed from the tube for regeneration of sulfur. It is a simple
method but during regeneration a lot of heat is released. Also the
dust packing contains a toxic component and the method is sensitive
to high water content of biogas.
Wood chips covered with iron oxide have a somewhat larger
surface to volume ratio than plain steel. Roughly 20g of H2S can be
bound per 100g of iron oxide chips. The application of wood chips
is very popular particularly in USA. It is a low cost product,
however, particular care has to be taken that the temperature does
not rise too high while regenerating the iron filter.
H2S can be adsorbed on activated carbon. The sulfur containing
carbon can then either be replaced with fresh activated carbon or
regenerated. It is a catalytic reaction and carbon acts as a
catalyst [5].
Liquid Phase Oxidation Process
This process is primarily used for the treatment of gases
containing relatively low concentration of H2S. It may be either:
(a) physical absorption or (b) chemical absorption.
In physical absorption process the H2S can be absorbed by the
solvents. One of the solvent is water. But the consumption of water
is very high for absorption of small amount of H2S. If some
chemicals like NaOH are added to water, the absorption process is
enhanced. But it forms sodium sulfide or sodium hydrosulfide, which
is not regenerated and poses problems of disposal.
Chemical absorption of H2S can take place with iron salt
solutions like iron chloride. This method is extremely effective in
reducing high H2S levels. The process is based on the formation of
insoluble precipitates. FeCl3 can be added directly to the digester
slurry. In small anaerobic digester system, this process is most
suitable. All other methods of H2S removal are suitable and
economically viable for large-scale digesters. By this method the
final removal of H2S is about 10ppm [5].
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Chapter 2.2: Scrubbing Techniques in Use A number of gas
upgrading technologies have been developed for the treatment of
natural gas, town gas, sewage gas, landfill gas etc. However, not
all of them are recommended for the application with biogas because
of price or environmental concerns. Water Scrubbing:
Water scrubbing is used to remove carbon dioxide but also
hydrogen sulphides from biogas since these gases are more soluble
in water than methane. The absorption process is purely physical.
Usually the biogas is pressurized and fed to the bottom of a packed
column where water is fed on the top and so the absorption process
is operated counter-currently.
Water scrubbing can also be used for selective removal of
hydrogen sulphide since hydrogen sulphide is more soluble than
carbon dioxide in water. The water which exits the column with
absorbed carbon dioxide and/or hydrogen sulphide can be regenerated
and recirculated back to the absorption column. The regeneration is
made by de-pressurising or by stripping with air in a similar
column. Stripping with air is not recommended when high levels of
hydrogen sulphide are handled since the water will soon be
contaminated with elementary sulphur which causes operational
problems. The most cost efficient method is not to recirculate the
water if cheap water can be used, for example, outlet water from a
sewage treatment plant [4].
Figure 1: Schematic flow sheet for water absorption with
recirculation for removal of carbon dioxide or hydrogen sulphide
from biogas [3]
Carbon Molecular Sieving: Molecular sieves are excellent
products to separate specifically a number of different gaseous
compounds in biogas. Thereby the molecules are usually loosely
adsorbed in the cavities of the carbon sieve but not irreversibly
bound. The selectivity of adsorption is achieved by different mesh
sizes and/or application of different gas pressures.
When the pressure is released the compounds extracted from the
biogas are desorbed. The process is therefore often called pressure
swing adsorption (PSA). To enrich methane from biogas
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the molecular sieve is applied which is produced from coke rich
in pores in the micrometer range. The pores are then further
reduced by cracking of the hydrocarbons.
In order to reduce the energy consumption for gas compression, a
series of vessels are linked together. The gas pressure released
from one vessel is subsequently used by the others. Usually four
vessels in a row are used filled with molecular sieve which removes
at the same time CO2 and water vapour. After removal of hydrogen
sulphide, i.e. using activated carbon and water condensation in a
cooler at 4C, the biogas flows at a pressure of 6bar into the
adsorption unit. The first column cleans the raw gas at 6bar to an
upgraded biogas with a vapour pressure of less than 10ppm H2O and a
methane content of 96 % or more.
In the second column the pressure of 6bar is first released to
approx. 3bar by pressure communication with column 4, which was
previously degassed by a slight vacuum. In a second step the
pressure is then reduced to atmospheric pressure. The released gas
flows back to the digester in order to recover the methane. The
third column is evacuated from 1 bar to 0.1bar. The desorbed gas
consists predominantly of carbon dioxide but also some methane and
is therefore normally released to the environment. In order to
reduce methane losses the system can be designed with recirculation
of the desorbed gases.
The product gas of column 1 is monitored continuously for CH4 by
an infrared analyser. If the required Wobbe index is not maintained
the gas flows back to PSA. If the methane content is high enough,
the gas is either introduced into the natural gas net or compressed
in a 3 stage compressor up to 250bar. Continuous monitoring of a
small-scale installation (26m3/hr) demonstrated excellent results
of gas cleaning, energy efficiency and cost.[3]
Figure 2: Schematic flow sheet for upgrading of biogas to
vehicle fuel standards with carbon molecular sieves [3]
Membrane Separation:
There are two basic systems of gas purification with membranes:
a high pressure gas separation with gas phases on both sides of the
membrane and a low-pressure gas liquid absorption separation where
a liquid absorbs the molecules diffusing through the membrane.
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High Pressure Gas Separation: Pressurized gas (36bar) is first
cleaned over for example an activated carbon bed to remove
(halogenated) hydrocarbons and hydrogen sulphide from the raw
gas as well as oil vapour from the compressors. The carbon bed is
followed by a particle filter and a heater. The membranes made of
acetate-cellulose separate small polar molecules such as carbon
dioxide, moisture and the remaining hydrogen sulphide. These
membranes are not effective in separating nitrogen from
methane.
The raw gas is upgraded in 3 stages to a clean gas with 96 %
methane or more. The waste gas from the first two stages is
recycled and the methane can be recovered. The waste gas from stage
3 (and in part of stage 2) is flared or used in a steam boiler as
it still contains 10 to 20 % methane.
First experiences have shown that the membranes can last up to 3
years which is comparable to the lifetime of membranes for natural
gas purification, a primary market for membrane technology, which
last typically two to five years. After 1 years permeability has
decreased by 30 % due to compaction. The clean gas is further
compressed up to 3.600 psi (250bar) and stored in steel cylinders
in capacities of 276m3 divided in high, medium and low pressure
banks. The membranes are very specific for given molecules, i.e.
H2S and CO2 are separated in different modules. The utilization of
hollow-fiber membranes allows the construction of very compact
modules working in cross flow [3].
Gas Liquid Absorption Membrane:
Gas-liquid absorption using membranes is a separation technique
which was developed for biogas upgrading only recently. The
essential element is a microporous hydrophobic membrane separating
the gaseous from the liquid phase. The molecules from the gas
stream, flowing in one direction, which are able to diffuse through
the membrane, will be absorbed on the other side by the liquid
flowing in counter current. The absorption membranes work at
approx. atmospheric pressure (1bar) which allows low-cost
construction. The removal of gaseous components is very efficient.
At a temperature of 25 to 35C the H2S concentration in the raw gas
of 2% is reduced to less than 250ppm. The absorbent is either Coral
or NaOH.
H2S saturated NaOH can be used in water treatment to remove
heavy metals. The H2S in Coral can be removed by heating. The
concentrated H2S is fed into a Claus reaction or oxidised to
elementary sulphur. The Coral solution can then be recycled. CO2 is
removed by an amine solution. The biogas is upgraded very
efficiently from 55% CH4 (43% CO2) to more than 96% CH4. The amine
solution is regenerated by heating. The CO2 released is pure and
can be sold for industrial applications [3].
Adsorption Using Iron Oxide and Activated Carbon
Hydrogen sulphide reacts easily with iron hydroxides or oxides
to iron sulphide. The reaction is slightly endothermic; a
temperature minimum of approximately 12C is therefore required to
provide the necessary energy. The reaction is optimal between 25
and 50C. Since the reaction with iron oxide needs water the biogas
should not be too dry. However, condensation should be avoided
because the iron oxide material (pellets, grains etc.) will stick
together with water which reduces the reactive surface.
The iron sulphides formed can be oxidised with air, i. e. the
iron oxide is recovered. The product is again iron oxide or
hydroxide and elementary sulphur. The process is highly exothermic,
i.e. a lot of heat is released during regeneration. Therefore,
there is always a chance that the mass is self-ignited. The
elementary sulphur formed remains on the surface and covers the
active iron oxide surface. After a number of cycles depending on
the hydrogen sulphide concentration the iron oxide or hydroxide bed
has to be exchanged. Usually an installation has two reaction beds.
While the first is desulphurising the biogas, the second is
regenerated with air.
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The desulphurisation process works with plain oil free steel
wool covered with rust. However, the binding capacity for sulphide
is relatively low due to the low surface area.
Wood chips covered with iron oxide have a somewhat larger
surface to volume ratio than plain steel. Their surface to weight
ratio is excellent thanks to the low density of wood. Roughly 20
grams of hydrogen sulphide can be bound per 100 grams of iron oxide
chips.
The application of wood chips is very popular particularly in
the USA. It is a low cost product, however, particular care has to
be taken that the temperature does not rise too high while
regenerating the iron filter.
The highest surface to volume ratios are achieved with pellets
made of red mud, a waste product from aluminum roduction. However,
their density is much higher than that of the wood chips. At
hydrogen sulphide concentrations between 1.000ppm and 4.000ppm
totally 50 grams can be loaded on 100 grams of pellets. Most of the
German and Swiss sewage treatment plants without dosing of iron
chloride are equipped with an iron oxide pellet installation.
With PSA systems H2S usually is removed by activated carbon
doted with potassium iodide (KI). Like in biological filters in
presence of air which is added to the biogas, the hydrogen sulfide
is catalytically converted to elementary sulphur and water. The
sulphur is adsorbed by the activated carbon. The reaction works
best at a pressure of 7 to 8 bar and a temperature of 50 to 70C.
The gas temperature is easy to achieve through the heat formed
during compression. Usually, the carbon filling is adjusted to an
operation time of 4.000 to 8.000 hours. If a continuous process is
required the system consists of two vessels. At H2S concentrations
above 3.000ppm the process is designed as a regenerative system
[3].
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Chapter 3: Proposed Design
Figure 3: Proposed design of scrubber
Assumptions:
Meshed sheet is assumed as a plane cylinder i.e. we are assuming
that pressure will drop continuously along the length, which has to
be corrected using experimental results.
Laminar flow in the tubular region which is found correct after
calculation.
Repeating pattern of corrugation with a fixed height and pitch
as shown in Fig 4
Stripped water has no H2S or CO2
Efficiency of both blower and pump were taken to be 80%
each.
Design parameters:
Dimension of Absorber
Outer Diameter (OD) = 500mm
Inner Diameter (ID) = 500/3mm
Flow rate of biogas = 10,800 m3/day
Absorbing Solvent = Water (Fresh + Stripped)
Consumption of a given chemical in biogas= tD . NR . Gma1. x
Where,
Run Duration = tD
No. of runs =NR
Gma1 = mass flow rate of biogas
x = % of the respective constituent of biogas
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Area of cross-section of absorption = 0.117 m2
Hydraulic Diameter= 4Acs/L
Where Acs=Area of cross section
L= Wetted perimeter
Calculation of biogas exposed surface:
R - Radius of the outer cylinder r- radius of the shaft
Angle subtended by wetted portion at the centre
From Fig. 3
= - 2
Also, sin = r/R => = 2 sin-1(r/R)
Area under water = (Area of sector subtending angle at the
centre) (Area of two s)
= ( /2 )(R2) (R cos )r Substituting and , we get
Wetted Area = R2/2 R2 sin-1(r/R) r (R2 - r2 )
Taking R = 250 mm and r = 250/3mm
Wetted Area = 57,293.11 mm2
Cross Section exposed to Biogas (Acs) = Total Cross Section
Wetted Area
= 174,532.93 57,293.11 mm2
= 117,239.82 mm2 = 0.117 m2
Calculation of wetted perimeter (P)
Figure 4: Meshed sheet
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Length of one section of curve y = sin ( x/5) from 0 to 10 (i.e.
for pitch p)
210
0
1 dyl dxdx
= + (Formula for length of any curve for given interval)
102
0
1 (2.5cos )l dx= + Or l = 0.021 m
If Number of such meshed sheets are x then
R r = x (h+2) => 5003( 2)
xh
=+
Or x = 24
Diameter after ith sheet
Di = ID + i (h+2)
Now for ith sheet, perimeter of ith sheet = number of loops of
pitch p * pitch p
i id n p =
ii
dnp
=
Total wetted perimeter L = ni (2 l +2p)
Or 0
(2 2 ) ( 2)x
n
l pL ID n hp
=
+= + +
Or L = 112.288 m
So wetted area Aw = L*1 (as we are assuming length of the sheet
to be 1m)
Hydraulic Diameter Dh = 4Acs/L
Or Dh = 4.23 mm
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Chapter 3: Conclusion After entering all the mass transfer,
pressure drop, area and wetted perimeter calculation
equations etc in the MathCAD program (see Appendix) some initial
results have been calculated.
For our proposed design, with Inner diameter of wheel/Diameter
of the shaft (ID) =500/3mm and Outer diameter of wheel (OD) =500mm.
The corrugated sheets disc if dipped in to water up to 1/3rd of its
height, the area exposed to biogas (Acs) turns out to be 0.117 m2.
Since we are using meshed sheets as shown in figure 4, this entire
area wont be available for mass transfer. Thus on calculating the
actual area involved in mass transfer, we get the wetted area
density as Aw = 112.288 m2/m3.
In order to get desired concentration output of. CH4 = 95-98%,
CO2 = 3-5%, H2S < 10ppm we require some minimum flow rate of
water for the given flow rate of biogas (10,800 m3/day). This is
calculated from the mass transfer equations and found to be
889,729.92m3/day which is around 80 times the flow-rate of biogas.
Also the minimum contact area required to get this mass transfer
done was 0.205 m2.
On the other hand we propose to use atmospheric air which is
available cheaply to strip the rich solution of its CO2 and H2S
content. Working of stripper is the same as that of absorber, the
only difference being, that in absorber, the components of biogas
were getting absorbed in water while in stripper, components from
water were getting absorbed in air. We assumed the composition at
the outlet of stripper in order to get minimum flow rate of air
required. Here we assumed that CO2 and H2S both are absent from
stripped water. This stripped water is then re circulated in the
system. So we got the minimum flow rate of air required (28.95
mol/sec), for a given flow of water which is equal to outlet flow
rate of water.
To get the pressure drop across the tube, in order to get blower
power, we approximate the meshed sheet tube as a plain tube. We get
the hydraulic diameter for the given flow rate as 4.23mm and
Reynolds number as 641.591. Since its a tubular flow, knowing the
friction factor, we can get the pressure drop as 23.482bar and from
that we get the blower power as 458.626kW. Similarly we get the
pressure drop for stripper as 60.278 bar and blower power required
as 2.59 * 103 kW.
As we know the contact area required and area density, we can
then get length of the tube required. From this we can decide how
many wheels in series we would require, depending on the wheel size
constraints.
What needs to be done?
In the end we were able to design a basic working model for the
biogas scrubber. The next stage requires iterating the values
obtained from the stripper and the absorber and using them as
initial conditions. These results have not been optimized as we
have assumed some initial parameters here, so we need to calculate
those parameters at the outlet (of each stripper and absorber) from
the results obtained from the assumed case and then recalculate
them on the basis of these new parameters. After a few iterations,
the values obtained, would start converging, leading us to an
optimized result. Also makeup/re-circulated water has not
considered at this stage of the design process.
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References 1. www.wikipedia.com
2. www.sciencedirect.com
3. Biogas up gradation and utilization, Task 24 : Energy from
biological conversion of organic waste by IEA Bioenergy
4. O. Jnsson , M. Persson , Biogas as transportation fuel,
Swedish Gas Centre
5. S.S. Kapdi, V.K. Vijay, S.K. Rajesh, Rajendra Prasad, Biogas
scrubbing, compression and storage:perspective and prospectus in
Indian context, Renewable Energy xx(2004)
6. M. Gatrell, Electrochemical reduction of CO2 to hydrocarbons
to store renewable electrical energy and upgrade biogas
7. Treybal, R.E., Mass-Transfer operations 3rd ed. ISE.
8. International critical tables of numerical data, physics,
chemistry and technology ed. by E.W. Washburn.