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Biogas Purification using Coconut Shell Based Granular Activated Carbon by Pressure Swing Adsorption

Authors:
  • Amrita School of Agricultural Sciences

Abstract

Biogas is one of the most important renewable energy source. Biogas normally contains 55 – 65% methane (CH4) and 45 – 55% carbon dioxide (CO2) and trace amounts of hydrogen sulphide (H2S). Raw biogas cannot be used as vehicle fuel as it contains CO2 and H2S which lowers the calorific value and causes corrosion to the storage vessel respectively. Enriching the biogas by removing CO2 and H2S will significantly improve the quality of biogas. The application of biogas either as cooking fuel or engine fuel chooses the type of purification method. In this work biogas enrichment using coconut shell derived granular activated carbon as CO2 adsorbent by pressure swing adsorption (PSA) technique was investigated. The adsorption of CO2 by the adsorbent with varying pressure from 1 to 10 bar were experimentally examined. The results indicated that with increase in pressure adsorption also increases. Activated carbon (AC) showed adsorption affinity towards both CO2 and CH4 at lower pressure. As the pressure increases the material showed little higher affinity towards CO2 than CH4. The maximum methane content obtained after adsorption and CO2 reduction percent found to be 73.9 % and 44.7 % at 8 bar pressure respectively. The material showed lower biogas separation ability in terms of lower CH4 enrichment and lower CO2 reduction. So the selected AC cannot be used for biogas enrichment but it can be used for single gas adsorption.
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1178
Original Research Article https://doi.org/10.20546/ijcmas.2017.604.144
Biogas Purification using Coconut Shell Based Granular
Activated Carbon by Pressure Swing Adsorption
E. Akila*, S. Pugalendhi and G. Boopathi
Department of Bioenergy, AEC & RI, TNAU, Coimbatore- 03, India
*Corresponding author:
A B S T R A C T
Introduction
Biogas from anaerobic digestion of waste
material is a great source for future energy
needs. It is a very important source of
renewable methane (Tippayawong and
Thanompongchart, 2010). Biogas has low
heating value (6.5 kWh/Nm3), which is
approximately half that of the natural gas
value (Bauer et al., 2013). Biogas can be
directly used for thermal applications or if it
is upgraded, it can be a used as a vehicle fuel
and can replace natural gas. After methane
enrichment and compression it can be used as
vehicle fuel just like CNG (Vijay et al., 2006).
So enrichment of biogas is needed to convert
the biogas into portable form.
A lot of processes are available for
enrichment of methane content in biogas by
removing significant amount of carbon
dioxide (CO2) and hydrogen sulfide (H2S).
Commonly CO2 removal processes also
remove H2S.
These include absorption into liquid (physical
/chemical), adsorption on solid surface,
membrane separation, cryogenic separation
International Journal of Current Microbiology and Applied Sciences
ISSN: 2319-7706 Volume 6 Number 4 (2017) pp. 1178-1183
Journal homepage: http://www.ijcmas.com
Biogas is one of the most important renewable energy source. Biogas normally contains 55
65% methane (CH4) and 45 55% carbon dioxide (CO2) and trace amounts of hydrogen
sulphide (H2S). Raw biogas cannot be used as vehicle fuel as it contains CO2 and H2S
which lowers the calorific value and causes corrosion to the storage vessel respectively.
Enriching the biogas by removing CO2 and H2S will significantly improve the quality of
biogas. The application of biogas either as cooking fuel or engine fuel chooses the type of
purification method. In this work biogas enrichment using coconut shell derived granular
activated carbon as CO2 adsorbent by pressure swing adsorption (PSA) technique was
investigated. The adsorption of CO2 by the adsorbent with varying pressure from 1 to 10
bar were experimentally examined. The results indicated that with increase in pressure
adsorption also increases. Activated carbon (AC) showed adsorption affinity towards both
CO2 and CH4 at lower pressure. As the pressure increases the material showed little higher
affinity towards CO2 than CH4. The maximum methane content obtained after adsorption
and CO2 reduction percent found to be 73.9 % and 44.7 % at 8 bar pressure respectively.
The material showed lower biogas separation ability in terms of lower CH4 enrichment and
lower CO2 reduction. So the selected AC cannot be used for biogas enrichment but it can
be used for single gas adsorption.
Ke ywords
Biogas purification,
Activated carbon,
Pressure swing
adsorption, Carbon
dioxide, Methane.
Accepted:
12 March 2017
Available Online:
10 April 2017
Article Info
Int.J.Curr.Microbiol.App.Sci (2017) 6(4): 1178-1183
1179
and chemical conversion. Selection of the
appropriate process for a particular
application depends on the scale of intended
operation, composition of the gas to be
treated, degree of purity required and the need
for CO2 recovery (MENS report, 2001).
Adsorption is one of the most economical
methods of CO2 separation from biogas that
has been commercialized (Grande, 2011).
Among the technologies for adsorption, the
pressure swing adsorption (PSA) has gained
interest in the separation and capture of CO2,
due to its reduced costs in energy and
resources, when compared to conventional
separation methods, such as absorption and
distillation (Kapdi et al., 2005; Huang et al.,
2006).
PSA provides high efficiency in terms of high
methane enrichment. In PSA maximum of
99% pure methane can be achieved in the
pressure range of 4 to 8 bar (Urban et al.,
2009). This technique works on the principle
of physical adsorption.
The gas molecules will be adsorbed on the
adsorbent surface under pressure and when
the pressure is reduced the adsorbed gases
will be released. The success of this
technology lies in choosing the appropriate
adsorbent material which selectively adsorb
the particular gas species of concern. This
posts the requirement of efficient and cost
effective adsorbent that is made from locally
available material. Activated carbon is one of
the most widely used adsorbents because of
its higher adsorptive capacity (Hung et al.,
2005). AC has high surface area and porosity
which makes it suitable for variety of
applications.
The aim of this work was to test the coconut
shell derived activated carbon which is
available commercially for its ability on
biogas purification and also to test the CO2
reduction capability of that material.
Materials and Methods
Selection and characterization of adsorbent
Selection of the adsorbent material is based
on the properties like high surface area, total
pore volume and high adsorption ability.
Based on the above properties, coconut shell
based granular activated carbon was selected
as the adsorbent which was procured from
The Jacobi Carbons India Private Limited,
Coimbatore. The specification of the material
was presented in table 1. From table 1 it is
clear to see that the porosity of AC is 42%
and this pore volume mainly consists of
mesopores and micropores. For PSA design
and operation the porosity should be in the
range of 0.3 to 0.5 (Jain et al., 2003). This
property suggests that the material can be
used for gas adsorption process. The surface
morphology was studied using Quanta 250
electron microscope, FEI, Netherlands.
Experimental Setup
The adsorption experiments were performed
in a lab scale PSA column with height and
diameter of 1 m and 0.3 m respectively.
Packing height of the column was 0.6 m. Pall
rings was provided in alternate layers in the
PSA column with the adsorbent for easy
distribution of gas over the cross sectional
area of the column. Pall rings covers half of
the total packing volume and it was provided
in alternate layers with the adsorbent. The
packing height of 0.6 m was subdivided into 6
parts each with the height of 0.1 m. For
packing the materials inside the column and
to ensure partitioning of the packing
materials, porous plates with 2 mm diameter
holes were provided in between the materials
at every 0.1 m intervals, schematic of the
setup shown in figure 1. The biogas which
needs to be treated compressed to desired
pressure and sent through the bottom and
enriched gas was collected at the top of the
column. The experiment was carried out with
Int.J.Curr.Microbiol.App.Sci (2017) 6(4): 1178-1183
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the biogas inlet pressure range of 1 to 10 bar
and at a room temperature of 30oC. The
fraction of gas components after adsorption
was analyzed using GC, Nucon 5765. To
evaluate the adsorption of CO2 on the
adsorbent, percent of CO2 reduction was
calculated as follows:
(CO2 content of the raw biogas - CO2 content of the scrubbed biogas)
CO2 reduction, % = ----------------------------------------------------------------------------------- × 100
CO2 content of the raw biogas
Results and Discussion
Surface characterization by SEM
SEM image of selected AC is shown in figure
2. It can be seen from the micrographs that
the external surface of the adsorbent has lot of
holes and cracks with varying in size. The
surface is uneven and small size pores covers
maximum of its pore volume than large pores.
It can be inferred from SEM image that
material is suitable for gas adsorption
purpose.
Effect of biogas inlet pressure on CO2
reduction
The experiment for analyzing the biogas
purification ability of AC using PSA system
was carried out with the pressure range of 1 to
10 bar. Table 2 shows the percentages of CO2
and CH4 gases after adsorption. The biogas
initially had 47 % of CO2 and 51 % of CH4.
From 1 to 8 bar there was a slight increase of
CH4 %. So with increase in pressure, CH4
enrichment also increases. After 8 bar there
was a decrease in CH4 enrichment. Initial
lower methane content may due to
insufficient pressure for the adsorption of CO2
into the pores of the material and if the
pressure is increased adsorption of CO2
molecules into the pores of material increases.
But when the pressure further increases the
adsorbed gas may be carried away with the
outlet gas so the CH4 content in the outlet gas
decreases. In this process, maximum methane
content of 73.9 % was achieved at 8 bar
pressure.
Figure 3 shows the CO2 reduction percent
with respect to biogas inlet pressure. As the
pressure increases CO2 reduction also
increases. Minimum and maximum CO2
adsorption found to be 11% and 45% at 1 bar
and 8 bar respectively. Low reduction of CO2
may due to the fact that both CO2 and CH4
were absorbed by the material (Rios et al.,
2013) and when the pressure increases, the
material showed slight increase in adsorption
of CO2 than CH4.
Table.1 Specification of AC
Properties
Value
Form
Granules
Size
2.5 mm
Pore size
15 to 20 Å
Bulk density
484 kg.m-3
Porosity
0.42
Int.J.Curr.Microbiol.App.Sci (2017) 6(4): 1178-1183
1181
Table.2 Final CH4 content after adsorption (Initial CO2 = 47%, CH4 =51%)
Pressure, bar
CH4 final, %
1
57.8
2
60
3
63.1
4
65
5
66.6
6
69.6
7
72.8
8
73.9
9
72.4
10
71.5
Fig.1 Schematic illustration of PSA unit
Fig.2 SEM image of AC
Int.J.Curr.Microbiol.App.Sci (2017) 6(4): 1178-1183
1182
Fig.3 Effect of biogas inlet pressure on CO2 reduction
There is an initial and preferential filling of
high-energy sites for which the more strongly
adsorbed component (CO2) is even more
preferred in the competition for the sites than
when competing for energetically weaker
adsorption sites (Rios et al., 2013). The
competitive nature of the two gas species was
seen at higher pressure. Since CO2 is more
strongly held by the adsorbent, it tends to
displace the previously adsorbed CH4 (Foeth
et al., 1994).
During adsorption the molecules dimension
play an important role. The pore size of the
material cavities is selective factor for the
adsorbed molecules. As the pore size of
activated carbon (15 to 20 Å) is greater than
the molecule diameter of the gas species CO2
(3.4 Å) and CH4 (3.8 Å) the selective
adsorption was not accomplished and the
material adsorbs both CO2 and CH4 results in
less methane content in the product gas
(Bonenfant et al., 2008).
In conclusion, the study was conducted to
evaluate the suitability of AC for biogas
purification. The material showed adsorption
affinity towards both CO2 and CH4 at all
pressure. Even though the material exhibited
slightly higher adsorption of CO2 than CH4,
the maximum CO2 reduction was only 45%.
This indicates that the material has lower
ability in purification of biogas. But the
selected material showed better adsorption
capacity of gases so it can be used for areas
where single gas adsorption is required like
CO2 capturing or for methane storage.
References
Bauer, F., Persson, T., Hulteberg, C. and
Tamm, D. 2013. Biogas upgrading
technology overview, comparison and
perspectives for the future. Biofuels,
Bioproducts and Biorefining, 7(5): 499-
511.
Bonenfant, D., Kharoune, M., Niquette, P.,
Mimeault, M. and Hausler, R. 2008.
Advances in principle factors
influencing carbon dioxide adsorption
on zeolites. Sci. Technol. Adv.
Materials, 9(1): 013007.
Foeth, F., Andersson, M., Bosch, H., Aly, G.
and Reith, T. 1994. Separation of dilute
CO2-CH4 mixtures by adsorption on
activated carbon. Separation Sci.
Technol., 29(1): 93-118.
Grande, C.A. 2011. Biogas upgrading by
pressure swing adsorption. INTECH
Open Access Publisher.
Huang, C.C., Chen, C.H. and Chu, S.M. 2006.
Effect of moisture on H2S adsorption by
Int.J.Curr.Microbiol.App.Sci (2017) 6(4): 1178-1183
1183
copper impregnated activated carbon. J.
Hazardous Materials, 136(3): 866-873.
Hung, Y.T., Lo, H.H., Wang, L.K., J.R.
Taricska, K.H. and Li, 2005. Granular
activated carbon adsorption, in: Wang,
L.K., Hung, Y.T and Shammas N.K.
(Eds.), Handbook of Environmental
Engineering, Humana Press, Totowa,
New Jersey, 3: 573634.
Jain, S., Moharir, A.S., Li, P. and Wozny, G.
2003. Heuristic design of pressure
swing adsorption: a preliminary study.
Seperation and Purification Technol.,
33(1): 25-43.
Kapdi, S.S., Vijay, V.K., Rajesh, S.K. and
Prasad, R., 2005. Biogas scrubbing,
compression and storage: perspective
and prospectus in Indian
context. Renewable Energy, 30(8):
1195-1202.
Rios, R.B., Stragliotto, F.M., Peixoto, H.R.,
Torres, A.E.B., Bastos-Neto, M.,
Azevedo, D.C.S. and Cavalcante Jr,
C.L. 2013. Studies on the adsorption
behavior of CO2-CH4 mixtures using
activated carbon. Brazilian J. Chem.
Engi., 30(4): 939-951.
Tippayawong, N. and Thanompongchart, P.
2010. Biogas quality upgrade by
simultaneous removal of CO2 and H2S
in a packed column reactor. Energy,
35(12): 4531-4535.
Urban, W., Lohmann, H. and Girod, K. 2009.
Technologies and cost of biogas
upgrading and feeding into the natural
gas grid, Oberhausen: Fraunhofer
Institute for Environmental, Safety and
Energy Technol., UMSICHT.
Vijay, V.K., Chandra, R., Subbarao, P.M.V.
and Kapid, S. 2006. Biogas
purification and bottling into CNG
cylinders: producing Bio-CNG from
biomass for rural automotive
applications. A paper presentation at
The 2nd Joint International Conference
on Sustainable Energy and
Environment (SEE), Bangkok,
Thailand.
How to cite this article:
Akila, E., S. Pugalendhi and Boopathi, G. 2017. Biogas Purification using Coconut Shell Based
Granular Activated Carbon by Pressure Swing Adsorption. Int.J.Curr.Microbiol.App.Sci. 6(4):
1178-1183. doi: https://doi.org/10.20546/ijcmas.2017.604.144
... Yang et al. [73] have studied the adsorption capacity of AC obtained from coconut shells by adsorption of CH 4 and CO 2 at 25 • C and 0-200 kPa. They prepared various ACs, including K-AC (carbonization in KOH solution), P-AC (carbonization in phosphoric acid solution), and W-AC (carbonization in a horizontal reactor and purified by nitrogen). ...
... Effect of biogas inlet pressure on CO 2 reduction[73]. ...
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