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IIUM Engineering Journal, Vol. 14, No. 2, 2013 Karimov et al.
145
THE ECONOMICS OF BIOGAS PRODUCTION
K
H
.S.
K
ARIMOV
1,2
,
M.A
BID
1
,
S.I.I
SLOMOV
3
,
N.H.K
ARIMOVA
3
AND
M.W.
A
L
-G
RAFI
4
1
GIK Institute of Engineering Sciences and Technology, Topi, Pakistan.
2
Physical Technical Institute of Academy of Sciences, Tajikistan.
3
Institute of Economics of Academy of Sciences of Tajikistan, Dushanbe, Tajikistan.
4
Department of Mechanical Engineering, Taibah University,
Al-Madina Al-Munawara, Saudi Arabia.
khasan@giki.edu.pk, abid@giki.edu.pk
ABSTRACT:
In this paper life-cycle cost analysis of three biogas digesters is presented.
Results show that the cost of biogas depends on the construction of digesters, sizes of
methane tank and possibility of heating of the slurry. Biogas and natural gas costs
calaculated are observed and found to be comparable. It is recommended that the biogas
digesters can be constructed and installed, in principle, for every family and there is no
need to built long gas pipe lines.
ABSTRAK:
Kertaskerja ini membentangkan analisis kos kitar hayat tiga pencerna biogas.
Keputusan menunjukkan kos biogas bergantung kepada pembinaan pencerna, saiz tangki
metana dan kemungkinan pemanasan buburan. Pengiraan kos biogas dan gas asli diambil
kira dan ianya didapati setanding. Adalah disarankan pencerna biogas boleh dibina dan
dipasang secara teorinya, bagi setiap keluarga tanpa memerlukan pembinaan paip gas
yang panjang.
KEYWORDS:
biogas; digester; slurry; life-cycle costing; annualized life-cycle cost;
cost of biogas; natural gas
1. INTRODUCTION
Solar energy is practically an inexhaustible source and can satisfy energy needs of the
world for many centuries. Considerable attention has been paid in the industrially
developed and developing countries to the utilization of solar energy present in biomass
as a result of photosynthetic process [1-3]. In addition, environmental agencies demand
proper processing of organic wastes to control problems of infection from the utilization of
huge mass of dung from cattle-breeding farms. A recent trend in the power industry is the
production of small-scale biogas plants, satisfying the needs of one or several families
living in the countryside. However, in China, India and Sri Lanka large factories for the
production of biogas that can supply electricity to the population of the whole village or
settlement have been constructed. In addition, recently the most successful designs have
appeared in Western Europe. An increasing trend of biogas technology is also observed in
the developed countries. For example in Germany alone, during the last 17 years, the
numbers of biogas plants have increased from 100 to 3500, i.e. 35 times [2], which is
indeed a substantial increase (Fig. 1).
In Tajikistan, development of biogas technology is in its initial stages; though the
country has been importing natural gas from Uzbekistan at a very high cost ( US$300 per
1000m
3
). Therefore at present it is very important to accelerate the installation of biogas
digesters. For this, knowledge of economics of biogas technology will be very useful for
the bio gasification in remote and rural areas. In this paper life-cycle costing (LCC) and
IIUM Engineering Journal, Vol. 14, No. 2, 2013 Karimov et al.
146
annualized life-cycle cost (ALCC) approaches have been used to analyze the economics of
three biogas digesters installed in Tajikistan and Pakistan.
Fig. 1: Development of biogas industry in Germany from 1990 to 2006. [2]
2. BIOGAS DIGESTER INSTALLED IN VAHDAT DISTRICT OF
TAJIKISTAN
Biogas digesters were constructed with the financial support of international
organization ISAR and help of scientists of S.U. Umarov Physical Technical Institute of
the Academy of Sciences of the Republic of Tajikistan in a private farm in Vahdat district
(Fig. 2). This digester was made of metallic tanks of capacity 10m
3
and was filled up to
2/3 of the volume with active biomass. This digester was equipped with manometers,
safety gates and valves and was operated with all the safety precautions and regulations as
required for natural gas. Retention time was 18 days during the summer season. The
biogas product was used for cooking and electric power generation.
(a) (b)
Fig. 2: Biogas digester installed: (a) in Vahdat village of Tajikistan, (b) and gas stove.
3. BIOGAS DIGESTER INSTALLED IN RUDAKI DISTRICT OF
TAJIKISTAN
In the digester at Rudaki, methane tank and gas holder were mounted separately as
shown in Fig. 3. In the middle of the methane tank, a mixer of special design was built to
mix slurry. Methane tank of 10m
3
capacity was loaded with up to 80% by slurry.
Retention time was 30 days in the winter. Biogas from methane tank to the point of
consumption was transported through metallic pipes of 15-50mm diameter. Productivity of
the biogas from this digester was in the range of 6-9m
3
per day and was used for making
buns (Fig. 4a) in an oven and generation of electric power using a low power electric
0
1000
2000
3000
1990 1992 1994 1996 1998 2000 2002 2004 2006
Number of Biogas
plants
Year
IIUM Engineerin
g Journal, Vol. 14, No. 2, 2013
generator (Fig. 4b). The residue
used as high quality fer
tilizer for agriculture.
Fig. 3:
Biogas dige
(a)
Fig. 4: (a) Buns
prepared using biogas, (b) electric generator for p
power from biogas in Tajikistan.
4. SOLAR BIOGAS DIGESTER FABRICATED IN GIK INSTITU
OF PAKISTAN
Earlier several solar biogas digesters
of Pakistanand in
continuation
digesters. I
nvestigations of more efficient solar biogas digest
absorber heater
is presented here
production and decreases the retention time. The bi
tank with built-
in solar reverse absorber heater to utilize solar e
slurry prepared
from different organic wastes (dung, sewage, food w
digester is of a laboratory
scale
methan
e tank of size 0.8 m in diameter,
reverse absor
ber heater installed under the methane tank. The ab
blackened to increase absorption of solar irradiati
consists of horizontal axes cylindrical reflecting
horizonta
l absorber is a common element of the methane tank
cylindrical reflector was used with
o
uter surface of the digester was covered by thermo
polymer film. The methane tank was filled up to 70%
g Journal, Vol. 14, No. 2, 2013
147
generator (Fig. 4b). The residue
in the digester after the
production of biogas
tilizer for agriculture.
Biogas dige
ster installed in Rudaki district of Tajikistan.
(b)
prepared using biogas, (b) electric generator for production of electric
power from biogas in Tajikistan.
4. SOLAR BIOGAS DIGESTER FABRICATED IN GIK INSTITU
Earlier several solar biogas digesters
were designed and fabricated by
continuation
with its efforts in design and fabricate
nvestigations of more efficient solar biogas digester with built
is presented here
. T
he heating of slurry in biogas digester increases b
production and decreases the retention time. The biogas digester consists of
in solar reverse absorber heater to utilize solar energy for the heating of the
from different organic wastes (dung, sewage, food wastes
scale
and consists of a
horizontal axes cylindrical metallic
e tank of size 0.8 m in diameter,
2m in length and 1 m
3
in volume with a solar
ber heater installed under the methane tank. The absorber
blackened to increase absorption of solar irradiation. The solar reverse absorber heater
consists of horizontal axes cylindrical reflecting mirror and horizontal glass cover. A
l absorber is a common element of the methane tank and solar reverse heater.
cylindrical reflector was used with
a radius of 0.7 m and glazing area
of 1.9x0.7m
uter surface of the digester was covered by thermo insulated material, aluminum foil an
polymer film. The methane tank was filled up to 70% of
its
volume by organic wastes of
Karimov et al.
production of biogas
could be
roduction of electric
4. SOLAR BIOGAS DIGESTER FABRICATED IN GIK INSTITUTE
GIK Institute
solar biogas
er with built
-in reverse
he heating of slurry in biogas digester increases biogas
ogas digester consists of
a methane
nergy for the heating of the
astes
, etc.). The
horizontal axes cylindrical metallic
in volume with a solar
sorber
surface was
on. The solar reverse absorber heater
mirror and horizontal glass cover. A
and solar reverse heater.
A
of 1.9x0.7m
2
. The
insulated material, aluminum foil an
d
volume by organic wastes of
IIUM Engineering Journal, Vol. 14, No. 2, 2013 Karimov et al.
148
the GIK institute sewage (70%) and cow dung (30%). Figures 5and 6 show the schematic
diagrams and photograph of the solar biogas digester respectively. Elastic rubber tubes of
volume of 0.5m
3
were used as gas storage containers.. The performance of the digester
was investigated during the period (January-March, 2011). The solar irradiance incident to
the absorber, slurry’s temperature and ambient temperature were measured. Figure 8
shows slurry and ambient temperatures during 11 days in March 2011.
Fig. 5: Schematic diagram of solar biogas digester.
(a)
(b)
Fig. 6: (a) Solar biogas digester fabricated at GIK Institute of Pakistan, (b) Scrubbing
tower for removal of carbon dioxide from biogas.
Figure 7 showsthat the slurry temperature is higher than the ambient temperature. It was
found that using sewage and cow dung the retention time was almost 2 weeks and
approximately 0.8-1.0m
3
of biogas was produced daily. Concentration of methane in the
biogas on the average was approximately 74%. Biogas was up graded by the removal of
carbon dioxide, hydrogen sulphide and water vapor. Figure 6b showsthe scrubbing tower
for removal of carbon dioxide from biogas. This biogas digester may be used domestically
as well as for demonstrative/teaching purposes. In addition, based on the results achieved
it can be used for the construction of larger volume solar biogas digesters for use on
farms, especially located in remote areas.
Biogas
Slurry
Cylindrical
methane
Reflector
Support
The
inclined
glass cover
Reverseabsorber
heater
Absorber
Outlet
Biogas
Inlet Cylindrical methane tank
Slurry
Reverse absorber heater
Tap
IIUM Engineering Journal, Vol. 14, No. 2, 2013 Karimov et al.
149
Fig. 7: Sludge (T
S
) and ambient (T
a
) temperatures during11 days in March 2011.
5. COST ANALYSIS
The cost of bio-gas was estimated on the basis of LCC and ALCC approaches
described in [3] using the following expressions:
PW= Cr ɯ Pr
(1)
where
PW
is the present worth for a single future payment,
Cr
is a single future cost,
Pr
is a discount factor for a single future payment. In the case of repeating payments [3]:
PW = Ca ɯ Pa
(2)
where
PW
is the present worth for annual future payment,
Ca
is the annual future
cost,
Pa
is a discount factor for annual future payment. The
Pa
and
Pr
is determined as
[3]:
Pr = [(1+i)/ (1+d)]
N
(3)
and
Pa = [(1+i)/(1+d)]·{[(1+i)/(1+d)]
N
-1}/{[(1+i)/(1+d)]-1}
(4)
where
N
is the period of analysis,
i
is excess inflation and
d
is discount rate.
The annualized life-cycle cost ALCC = LCC /
Pa
(5)
Considering digester’s volume (
V
d
) of 6.7 m
3
with retention time (
t
r
) of 18 days, the
flow rate of fluid (
V
f
) of 0.37 m
3
day
-1
of the digester fluid is determined by [4] as:
V
f
= V
d
/ t
r
(6)
The mass of dry input
m
o
= V
f
ȡ
m
(7)
where
ȡ
m
is the density of the dry matter in the fluid (~ 50 kgm
-3
): m
o
= 18.6 kg day
-1
.
The daily produced biogas volume determined by [4] as;
V
b
= cm
o
(8)
where
c
is biogas yield per unit dry mass of whole input (0.2-0.4m
3
kg
-1
) and an
average value of
V
b
calculated using Eq .8 is 5.6 m
3
. To the first approximation, the
volume of daily produced biogas is equal to the digesters (or slurry’s) volume, depending
on the nature and composition of organic wastes.
Natural gas is composed of CH
4
(up to 98%), other components are (C
2
H
6
), (C
3
H
8
),
(C
4
H
10
), (H
2
), (H
2
S), (CO
2
), (N
2
), and (He). The standard accepted temperature and
pressure of natural gas is 0
o
C and 1 bar respectively. Figure8 shows large increase in
0
5
10
15
20
25
30
35
40
45
0 3 6 9 12
Degree Celcius
Days
Ts
Ta
IIUM Engineering Journal, Vol. 14, No. 2, 2013 Karimov et al.
150
natural gas price from $80 to $280/Thousand m
3
in USA during the period1980 to 2005.
In the first approximation, as an average, the cost of the natural gas is comparable to cost
of the methane (US $ 95/Thousand m
3
) obtained from biogas. Life-cycle costing
calculationsfor the three biogas digesters are given in Table 1.
Fig. 8: Production and cost of natural gas in USA.
For biogas digesters of Vahdat and Rudaki districts of Tajikistan and GIK Institute of
Pakistan, if a biogas digester produces gas 330 days in a year considering cleaning and
slurry loading, and retention time for biogas production, the total produced gas will be
1848 m
3
, 2640 m
3
and 330 m
3
respectively. From Eq. (9) the cost of 1000 m
3
of biogas
determined for Vahdat, Rudaki and GIKI areUS$ 66.4, 60 and 75.8 respectively ;
Cost of biogas = ALCC / total produced gas (9)
Considering 70% concentration of methane in the biogas produced at Vahdat and
Rudaki digesters and 74% at GIK Institute, the cost of methane extracted from biogas is
US $95, 86 and 102 per thousand m
3
, respectively, with pressure usually of 0.1-0.2 bar.
The cost of the biogas of the digester installed in Rudaki district is less than the installed in
Vahdat district. It may be, firstly, due to the presence of the mixer that plays positive role,
in the case of digester installed in Rudaki. Secondly, due to the presence of large volume
gas-holder as it is well known that the rate of biogas production increases if pressure in the
digester is lower (as in the case of the digester installed in Rudaki district) and decreases if
pressure increases (as in the case of the digester installed in Vahdat district). The cost of
biogas of the digester at GIK Institute is higher than installed in Vahdat and Rudaki
districts. It may be, firstly, due to the small volume (1m
3
) of the solar biogas digester with
respect to the digesters in Vahdat and Rudaki districts (10m
3
). Usually if the volume of the
biogas digester is larger due to the exothermic reaction at anaerobic process the slurry is
heated additionally and production of the biogas increases slightly [7,8]. At the same time
experiments that were conducted with solar biogas digester and ordinary digester of the
same volume of methane tank showed that in the case of solar biogas digester retention
period was 15% less and biogas volume produced was 20% higher [12]. Table 2 shows
technical parameters and costs of biogas of the investigated digesters.
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IIUM Engineering Journal, Vol. 14, No. 2, 2013 Karimov et al.
151
Table 1: Life-cycle costing calculation sheet for the biogas digester installed in Vahdat
district of Tajikistan, Rudaki district of Tajikistan and GIK Institute of Pakistan.
Vahdat
Tajikistan
Rudaki
Tajikistan
GIKI
Pakistan
System Description
:
Volume of methane tank (m
3
)
Volume of slurry in methane tank (m
3
)
10
7
10
8
10
0.8
Parameters:
Period of analysis (years)
Excess Inflation (
i
)
Discount Rate (
d
)
10
0
10%
10
0
10%
10
0
10%
And Capital Cost:
Hardware (methane tank, pipes,
monometers, etc): ($)
Transportation: ($)
Installation: ($)
Total: ($)
400
100
100
600
570
100
100
770
70
10
10
90
Operation and Maintenance:
Annual Cost ($)
Discount factor (Pa)
Present Worth($)
20
6.14
122.8
25
6.14
153
10
6.14
61.4
Fuel:
Annual Fuel cost ($)
Present Worth ($)
0
0
0
0
0
0
Replacements:
Items
Duration (Years)
Cost ($)
Pr
PW
Total ($)
Pipes
5
50
0.62
31
31
Pipes Mixer
5 5
50 30
0.62
0.62
31
18.6
49.6
Pipes
5
5
0.62
3.1
3.1
Total Life
-
Cycle Cost
($)
Annualization Factor (Pa)
Annualized Life-Cycle Cost (ALCC):($)
753.8
6.14
122.8
973
6.14
158
154.5
6.14
25
Table 2: Technical parameters of investigated digesters and costs of
methane extracted from biogas.
Sr
#
Digesters Volume of methane
tank (m
3
)
Cost of methane
$/Thousand (m
3
)
1 Vahdat district (Tajikistan) 10 95
2 Rudaki district (Tajikistan) 95 86
3 GIK Institute (Pakistan) 10 102
4 Natural gas (USA at 1980-
2005)
01 80-280
6.
CONCLUSION
The utilization of biogas that contains actually the solar energy reserved in the
biomass as a result of photosynthetic process is practically very important for developed as
well as developing countries.A recent trend in the power industry is the production of
small-scale biogas plants that can satisfy the needs of one or several families living in the
countryside.Life-cycle cost analysis of three biogas digesters showed that the cost of
biogas depends on the construction of digesters, the size of methane tank and the heating
of the slurry. It was found that the cost of methane extracted from biogas is in the range of
IIUM Engineering Journal, Vol. 14, No. 2, 2013 Karimov et al.
152
US$86-102 /thousand m
3
and is comparable with the cost of natural gas. Biogas digesters
can be constructed and installed, in principle, for every family and there is no need to
build long gas pipe lines.
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