Biogas may be presented below [2]. 1. Better utilization

Biogas is the raw product that is obtained from anaerobic
digestion. The biogas so obtained may be utilized in the following two ways 1.

1.    
 To
convert the biogas into heat and electricity via a Combined Heat and Power Generation
(CHP) unit.

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2.    
To upgrade the biogas so that it may be fed to
the national grid.

In the present report, the second approach i.e. “Upgradation
of Biogas to Biomethane” was selected due to the plethora of reasons. Some
amongst them may be presented below 2.

1.    
 Better
utilization of the gas at an efficiency that is in the vicinity of the zenith
efficiency of a hundred percent.

2.    
Modern techniques may be used to reduce the cost
involved in processing.

3.     It
is not difficult to get buyers for the product

4.     Higher
profits may be obtained by supplying it to the national grid.

Procedure

 

The biogas goes
through a vast range of processes before it gets upgraded to biomethane as
listed below.

1.    
Storage

2.    
De-sulpherization

3.    
Water vapour removal

4.    
CO2 removal

5.    
Odourization

 

STORAGE

 

For the storage of the Biogas, the Walker
process double membrane gas holder is used. The structural constitution of the
holder runs as the outer membrane designed of a textile material followed by an
adjustable inner membrane that expands as a larger volume of the gas enters the
storage facility. The unit has an average durability period of twenty years 3.

The storage unit is designed assuming a peak
flow of 806400m3/20 days (based on calculations). Thus the volume of
the unit obtained for a retention period of twenty hours is 33600m3/20
hours. For this volume, the diameter of the storage tank is obtained as 40.2 meters.
Subsequently, a tank with a diameter of 42m is chosen.

Due to more influx of the gas, there is a
possibility that the volume of the spherical shell may not be sufficient to
engulf the gas for a period of 24 hours. In order to ensure the availability of
sufficient volume of storage and keeping in mind the safety considerations, a flare
system is to be put in place to burn the excess amount of volume of gas
available. This may further be ensured by providing a clause in the contract mentioning
that the storage of the unit should not be more than 20 hours.

 

De-Sulphurization

 

The biogas contains a large amount of undesirable
substances such as hydrogen sulphide, water vapours etc. These substance may
pose a threat to the efficient and normal working of the equipment used in the
process i.e. to upgrade the biogas. The hydrogen sulphide present in the gas

stream combines with the water vapour present, to form sulphuric
acid. Thus it is imperative to make the gas free from sulphur and dry it 4.

In order to achieve this, the company has partnered with SH
Sulphtec to supply the SH Sulphpur- a biological trickling filter. The
trickling filter is made up of PP or PE material. The said filter was chosen,
for amongst other things, because its maintenance requirements were very low.  Also the consumption of energy and oxygen was
very low 5. Two such filters with a flow of 850m³/hour will be used

Water Vapor Removal

 

 The water vapor needs to be eliminated from the biogas because it may cause the machines
of the plant to wear and tear resulting in their deterioration 4, 6.

A condensation pot is used to remove the water from the gas. The
pot (COP-1000) to be used for this purpose is procured from ENNOX limited which
is known for specialization in such type of products. It works on the principle
of separation of water vapours utilizing temperature gradient.  The gas moves through the condensate pot and
by dint of a temperature gradient, the water vapours condense and get collected
at the bottom. They can then be discharged via a siphon. The advantage of this
method is that the maintenance is low and also the pressure losses are minimum 7.
Two of these condensation pots with a peak flow of 840m³/hour will be used. The
specification of the apparatus have been presented in the Table below:

Parameter

Value

Durchsatz

750-1200 m3/hr

Durchmesser

955 mm

Hohe

1800 mm

Anschlusse

250

The carbon di oxide removal is done to increase the calorific
value of the gas and also to make the composition of the gas as per the DVGW Code of
Practice.4

The technology chosen to remove CO2 is taken from Pentair Haffmans
solution.  It works on the principle of
separation of gases. The gas is fed in a chamber equipped with a membrane
separating unit by dint of a pressure gradient. When the high pressure biogas
is fed in the separating unit, CO2 differentiates out as the permeate
on account of low pressure. Thus the gas with a high percentage of methane is   obtained. For the 100 percent recovery of
methane Cryogenic technology would be used 8.

Odourization

 

The
sulphur free compound namely a mixture of Methyl Acetate, Ethyl Acetate and 2
ethyl 3 methyl pyrazine is used for odorization. It has been employed in the
german natural gas for more than 10 years. The
chapter 4.1.2 of FPrEn 16723-2 advises a threshold value of 30mg/m³ un the grid
(energie) 9.After obtaining the Biogas, the Calorific value of the gas is
to be calculated. The route generaly adopted  
for this purpose is the addition of Propane. After getting the required
calorific value post adjustment of the pressure the gas can be fed to the energy
grid.  Adjustment of Calorific Value  Addition of propane to increase the
Calorific value is required in many countries 6. Based on the German Standard
G260/G262 the Wobbe Index was calculated.

Using the calorific value for bio methane
as 36MJ/Nm3 10 and a specific gravity of 0.68, the value came out
to be 53.73.Adjustment of Pressure   The gas after membrane
separation by Pentair Haffmans solution is compressed to a pressure of about 4
bars. Thus it can be directly fed into the distribution network.Cost EstimationSince a number of processes are required to be completed for
the generation of the biogas, the cost involved for these processes is
different. A cost of 7cents per kWh for biogas generation is assumed here 12.
Moreover there are no laws in Germany regulating the price at which the
Biomethane would be sold to the feeder. Thus a reasonable price of 10 European
cents per kWh is chosen.13Based on the amount of biogas obtained i.e. 11,677,068m3/a, a
methane content of  7,006,240.8 m3/a is
calculated (based on a 60% methane content by volume)

Assuming the biomethane obtained to be 98% pure the amount of
Biomethane is 6,866,115.98 m3/a. Based on the aforementioned values a profit of
2,076,313.47 million/a is obtained.References 1 Wilkie, C., A.,(2017, April 24) Biogas a renewable
biofuel. Retrieved from http://biogas.ifas.ufl.edu/uses.asp 2 Biomethane into the Gas Network: A Guide for Producers.
(2009). ebook p.10. Available at: http://www.organics-recycling.org.uk/uploads/category1060/Biomethane
into the Gas Grid a Guide for producers.pdf Accessed 29 Jan. 2018.3 Double membrane gas holder, Walker process equipment, Division of
McNish corporation Available
at: http://www.walker-process.com/pdf/TB6258.5.pdf    Accessed 29 Jan 2018 4 WEITHÄUSER,
M., SCHOLWIN, F., FISCHER, ER., GROPE, J., WEIDELE, T., and GATTERMANN,
H. Gas processing and options for
utilization. In Guide to biogas from
production to use(5th ed., pp 115-138).
Fachagentur Nachwachsende Rohstoffe e. V. (FNR). 5    SH Sulphpur, SH
Sulphtec Available at:        
https://sulphtec.com/biogas-desulphurization/sh-sulphpur/accessed 29
Jan 2018 6 HOYER, K., HULTEBERG, C.,
SVENSSON, M., JERNBERG, J. and NØRREGÃ…RD, Ø. (2016). Biogas upgrading –
Technical Review. ebook Available at:
http://vav.griffel.net/filer/C_Energiforsk2016-275.pdf Accessed 29 Jan. 2018.7  Condensation Pot
COP 1000, Ennox Available at: http://ennox.co.uk/wp-content/uploads/2016/01/Ennox-UK-Ltd-Condensate-Pot-COP-1.pdf     Accessed on: 29 Jan 2018 8  Membrane and Cryogenic Technology,  Haffmans Biomethane and Green CO2 brochure –
English, Pentair Haffamns Available at https://foodandbeverage.pentair.com/en/products/haffmans-biogas
upgrading-technology  Accessed on: 29
Jan 2018  9 Source for Odourization
table: Institute, B.S., Standard in development: BS EN 16723-2 Natural
gas and biomethane for use in transport and biomethane for injection in the
natural gas network Part 2: Automotive fuel
specifications. 2016. 10 The propane enrichment of
biomethane. (2018). ebook Available at:
https://www.flogas.co.uk/uploads/asset_file/2_21_biomethane-1.pdf Accessed 29
Jan. 2018 11 PERSSON, M.,
JÖNSSON, O. and WELLINGER, A. Biogas Upgrading to Vehicle Fuel Standards and Grid Injection. ebook Available at: http://www.ieabiogas.net/files/daten-redaktion/download/publi task37/upgrading_report_final.pdf
Accessed 29 Jan. 2018.12 Grope, J. Ökonomische Analyse der
Nutzungsmöglichkeiten von Biomethan, 2013. Available online:
http://www.biogasundenergie.de/downloads/scholwin_publication_16.pdf (accessed
on 29 January, 2018).13 Hartmann, S., Wirth,
B., Niebaum, A., Dohler, H., Keymer, U., and Reinhold, G. Economics. In Guide to biogas from production to use(5th
ed., pp 159-178). Fachagentur Nachwachsende Rohstoffe e. V. (FNR).   ,