Majlesi Journal of Mechatronic Systems
Vol. 3, No. 3, September 2014
Optimization Actions in Electrical Energy Generation of Biogas
Ahmad Reza Taheri Asl 1, Amir Hossein Zaeri2, Behzad Bahraminejad3, Foroogh Torki4
1- Department of Electrical and Computer Engineering, Islamic Azad University, Majlesi branch, Iran
E-mail: taheri_1977@yahoo.com
2- Department of Electrical and Computer Engineering Islamic Azad University, Majlesi branch, Iran
E-mail: Amzaeri@yahoo.com
3- Department of Electrical and Computer Engineering Islamic Azad University, Majlesi branch, Iran
E-mail: combahraminejad@gmail.com
4- Department of Electrical and Computer Engineering, Yazd University, Iran
E-mail: f_torki@stu.yazduini.ac.ir
Received: May 2014
Revised: July 2014
Accepted: August 2014
ABSTRACT:
Rising energy prices and growing energy technologies are encouraging consumers and governments to investment in
renewable energy sources. Biogas is a kind of these renewable energies sources which have been considered in
electrical energy generation. Biogas can be obtained of different natural wastes like municipal sewages, animal wastes,
agricultural wastes and industrial wastes. So biogas recovery systems can be established in all places where can access
to mentioned resources. This shows simplicity, abundance and accessibility of this type of energy generation. This
paper has focused on different parts of power generation from biogas-municipal wastewater system and has been
introduced some important and simple actions that can increase efficiency and thus the electricity generation.
KEYWORDS: Municipal Wastewater, Biogas, Power Generation, Efficiency.
1. INTRODUCTION
Biogas operations have numerous advantages including
production clean heat and electricity, reducing the
impact of organic wastes on the environment (i.e.
reduced greenhouse gases and lower impact on water
sources and courses) and enhancing the value of
residual products. [1]. Power generation of biogas has
numeral advantages that attract international attention
as an effective source in both rural and urban areas.
Some of these advantages are: decreasing environment
pollutions which wastes can spread around and
avoiding of ground water polluting, decreasing
greenhouse gas emissions in atmosphere because of
more less usage of fossil fuels, reducing amount of
fossil fuels consuming and etc.[2]
Biomass is the organic matter available on a renewable
basis. Biomass includes forest and mill residues,
agricultural crops and wastes, wood and wood wastes,
animal wastes, livestock operation residues, aquatic
plants, fast-growing trees and plants, and municipal and
industrial wastes. [1,3]
In Iran, according to a study that was conducted at the
Department of Energy, Biomass potential is estimated
as 74 million barrels of crude oil for agricultural and
forest waste, 36 million barrels of crude oil for animal
waste, 15 million barrels of crude oil for municipal
waste, 5.5 million barrels of oil for food waste and 2
million barrels of crude oil for municipal wastewater.
Therefore, according to 6.4 million m3 of municipal
and industrial wastewater in the country we can say
that potential to produce biogas from municipal
wastewater in the country is high. [4], [5]. So if we use
the anaerobic wastewater treatment process, produced
biogas would be approximately 107.8 to 245 million m3
(for cities over 100 thousand population). Due to
average efficiency of power generation systems of
biogas, which is about 40 percent, this amount of
biomass could produce about 22.5 to 51 million kWh of
electricity per year. It means that municipal wastewater
of country can play role of a 2.6 to 5.8 MW
PowerStation.
So rising amount of generated power will be accessible
if we can use some operations to increase system
efficiency. To reach this aim we must study all parts of
the system and understand reactions between each part
on another. In this paper based on operation of different
parts of power generation of municipal wastewater
system several efficient and simple operations are
proposed to increase system efficiency.
In this article at first some important sections of the
system are introduced, and then several simple and
effective actions to increase system efficiency are
offered.
7
Majlesi Journal of Mechatronic Systems
Vol. 3, No. 3, September 2014
2. AN OVERVIEW ON POWER GENERATING
SYSTEM OF BIOGAS
The diffrence between utilization of biogas from
municipal wastewater and other renewable energy
sources can be sumerized in four main aspects, energy
production, healthy environment, rich fertilizer and raw
materials in covers.
details and compounds of produced Biogas from
municipal wastewater digestion approximatly are
shown in Table 1 (it is important to note that each of
these parameters can be varied due to changes in the
composition of income sewage) [6]
Table 1. constituents and characteristics of a sample
produced biogas from wastewater a [6]
55-70
%methane,
30-45%
Composition
carbon dioxide, and trace
amounts of other gases.
Energy content
6 to 6.5 kW/m3
0.6 to 0.65 liters of fuel per m3 of
Fuel equivalent
biogas
Explosion limits
6 to 12 percent biogas in air
Combustion
650 to 750 ° C
temperature.
Critical pressure
75 to 89 bar
Critical
82.5 ° C
temperature
Normal density
1.2 kg/m3
Rotten eggs (hydrogen sulfide
odor
odor)
In general, regardless of the source of biogas
production, generation and use of biogas process can be
seen in Figure 1.
Fig. 1. Constituent parts of power generating system
from biogas
According to this figure1, it can be seen that the
mechanical and electrical energy required in every
place and process can be achieved by installing the
appropriate equipment. It means that thermal energy
can be created by burning biogas and convert it into
required mechanical and electrical energy in industries,
farms, and any other place.
In other words, a wastewater treatment plant where
produces biogas and then electricity will be produced
from biogas is comprised of 10 units that can be seen in
figure 2. [4]
Fig. 2. Schematic graph of power and biogas
generation in wastewater refinery process [4]
8
Majlesi Journal of Mechatronic Systems
Units 1 and 2 which collectively are called primary
treatment are responsible of suspended solids removal
from wastewater and preparation sewage for entering to
secondary treatment. Sections of the primary treatment
include
trash-stuck,
sedimentation,
flotation,
neutralization and balancing. The factor that is
important in this unit is the amount of Biological
Oxygen Demand (BOD) in incoming sewage.
According to what will be presented in the next section
control of this factor can increase system efficiency.
[7], [8], [9]
Unit 3, Secondary treatment, is includes all biological
treatment processes which are done in both aerobic and
anaerobic treatment. Conventional methods of
secondary treatment of sewage include: Activated
sludge method, Continuous aeration and Aerated
lagoons.
Unit 4, digester, is the most important part of the biogas
production system. Because of most main factors to
control quality and quantity of biogas, are available in
this unit. In other words, by controlling these factors in
this unit, quality and volume of biogas and electricity
can be administrated. Digesters are divided into two
types, Aerobic and anaerobic, that anaerobic type is
used in the biogas production from municipal
wastewater. [8]
Anaerobic digester is a tank sealed which mud is
entered to it and heated. In the absence of oxygen,
anaerobic bacteria grow by consuming solid waste
sectors and produce methane and carbon dioxide
(biogas). Efficiency of anaerobic digesters depends on
providing appropriate environment for bacteria and
feeding them. [8]
Units 5 and 6 are responsible for guarantying quality
and pressure of biogas before entering to power
generation unit. Currently, due to inadequate controls
in this unit, instability in power generation is seen.
Some simple actions are suggested in next section to
increase stability and efficiency of power generation.
[7]
Unit 6 and 7 are consisting of one or more gas engines
and convertor devices to generate electricity. It's
obvious that efficiency of engines and convertors can
affect the efficiency of the entire system.
3. SIMPLE ACTIONS TO INCREASE SYSTEM
EFFICIENCY
As mentioned in the previous section biogas production
systems from municipal wastewater which have already
been designed and implemented have flaws and defects
that greatly reduce system efficiency. In this paper
some strategies are presented to overcome these
deficiencies.
3.1. Adding extra equipment in digestion mixture
process
Vol. 3, No. 3, September 2014
As mentioned in the previous section, the efficiency of
anaerobic digester depends on suitable environment
and nutrients for bacteria. Since the bacteria have no
ability to move themselves, so mixing can be effective
in increasing the efficiency of anaerobic digester.
Now the mixing operation is accomplished by a mixer
which is controlled by a motor with variable speed. But
sometimes it's not enough for mixing. For example,
when temperature of the sludge of digester decreases,
because of cooling down the environment or entering
cold sewage from treatment plant, bacterial growth rate
reduce and rate of biogas production decrease.
Therefore, side operations such as heating content of
digester and injecting obtained gas into the digester
could make mixing operations more effective.
Unavailable or insufficient food for bacteria is limiting
activity of bacterial and PH Value of the digester
contents goes to acidic environment. Thus measuring
the PH can be an appropriate metrics for controlling
mixing operation. Increasing speed of mixers can be
effective in primary controlling the process. But if it is
not effective, lateral and auxiliary operations can be
used to improve the bacteria environment. One of
which is the addition of buffering materials.
One factor in this case is to increase the bio
temperature of the bacteria. Raising temperature of
digester in emergency situations by lateral processes
can help to reduce its acidity. So temperature of some
points inside the digester is measured by some sensors
and calculated subtraction of them to predict
temperature changes of digester environment in future.
So before creating acidic environment, temperature of
the digester begins to rise. Therefore, use of heat
exchangers in the digester is proposed. These
exchangers can be selected from two central and side
Heat exchangers.
It is noteworthy that in order to recovery exhaust heat
of gas engines and reduce energy consumption, output
steam or hot water of gas engine (is called CHP unit)
can be used.
Another effort which can be effectively in improving
the mixing process is injection the obtained gas into the
digester environment. If bacteria living environment is
far from prepare situation, amount of gas produced by
them will be decreasing. This causes that gas pressure
in top of the digester tank decline. While for receiving
gas from the digester continuously (which is one of the
main objectives) pressure at top of the digester should
be always positive and higher than a certain level. The
pressure much less means that environment is not
suitable for bacteria. Therefore, ambient air is
introduced into the digester and causing the digester
goes out of anaerobic state. So the growth of bacteria
and methane-making process will disrupt.
Therefor pressure changes at top of digester can show
PH changes. One of simple actions to increase the gas
9
Majlesi Journal of Mechatronic Systems
Vol. 3, No. 3, September 2014
pressure at top of the digester and control PH will be
injecting into the digester from the bottom. For this
purpose, a storage tank of gas can be used before the
gas refinery unit. By creating a return way of the tank
to the bottom of digester we can inject gas into the
sludge in digester when necessary.
This suggestion can be seen in figure 3.
Fig. 4. Locating a secondary tank to storage sludge
3.3. Implementing an automatic control system
between wastewater treatment, biogas production
and electricity generation units
Fig. 3. equipment Location to assist the mixing process
3.2. Adding a storage tank next to the digester
Various factors may affect amount of produced gas by
the bacteria in the digester. The most important of them
are retention time of sludge in the digester and sludge
concentration inside the digester. If these two factors
could be close to the ideal conditions, efficiency of gas
production will be increased.
It is possible that during a period of time amount of
wastewater entering the plant increase. So amount of
sludge to digester enters in much higher speeds.
Therefor digester sludge has to remove before the
desired retention period, 10 days, from the digester and
forcibly replaced with fresh, non-processed sludge.
This leads to exclude the processed sludge, which is
ready to produce gas, from the process before reaching
the peak gas production. This will reduce the efficiency
of the digester and biogas systems. On the other hand,
if for various reasons, such as rainfall, the
concentration of sludge in the pools before digester
decreases, the diluted sludge will enter to the digester
and gas production will greatly reduce. It can also
reduce efficiency of system like the previous operating.
A simple solution that is suggested is storing rich and
processed sludge in a storage tank for use in emergency
situations.
Figure 4 shows location of devices for storage slog.
10
Biogas production systems which are already being
used are not able to control the process instantaneous
and complete and don't have full-duplex and complete
connection with the collection system and wastewater
treatment. While the interaction between these two
systems, and more electrical energy generation system
by biogas is very important to increase efficiency and
reduce human errors. [9], [10]
So by implementing an online and real-time monitoring
system, In addition to continuous sampling, analysis
samples and storage results in long-term memory, we
can send the results in adjustable intervals through a
variety of online communication medias to central
control and monitor point.
3.4. Increasing power plant efficiency
In addition to the solutions that have already been
presented in different parts of biogas production
system, some actions are that can improve efficiency of
power generation system. One of the easiest and most
inexpensive ways is cooling the incoming air to the gas
turbines. By an absorption chiller to cool the intake air
can increase the area under the graph of pressure vs.
volume and increase plant efficiency.
These actions are shown in figure 5.
Fig. 5. Schematic of increasing power plant
Majlesi Journal of Mechatronic Systems
Vol. 3, No. 3, September 2014
4. CONCLUSION
In this paper we propose some simple and low cost
instruments to optimize power generation from
wastewater system. These actions can be used in all
parts of the system: wastewater refinery plant, biogas
production of wastewater sludge and power generation
of biogas. These help power generation process in
critical conditions to repair itself faster.
REFERENCES
[1] Report for Alberta Agriculture, Food and Rural
Development,
"Biogas
Energy
Electricity
Generation and Interconnection to the Power
Grid", 2006.
[2] Taheri Asl, reihani, Naderi Fasarani, Mostafavi,
“Potentiometer, made by biogas-burning power
plant in sewage treatment plants” the 9th
international Energy Assembl.y.
[3]. Bodík, S., Sedláèek, M., Kubaská and M., Hutòan,
“Biogas Production in Municipal Wastewater
Treatment Plants –Current Status in EU with a
Focus on the Slovak Republic”, Original scientific
paper, Slovak University of Technology, July 27, 2011.
[4] A., Taheri Asl, M., Malekiha, K., Movassaghi and F.,
Adani., “New Method for Biogas Generation from
Municipal Wastewater Treatment Plant of Isfahan
City, Iran” Eurasia 2014 Waste Management
Symposium. Istanbul, Turkey.
[5] M., Adl, Gh.A., Omrani.. "Background of biogas
technology and its recent developments in Iran",
International Journal of Global Energy Issues, Vol. 29,
No. 3, pp. 273-283, 2003.
[6]. Z., Hanjie..“Sludg Treatment to Increase Biogas
Production. Trita-Lwr”, Degree Project , pp.10-20,
August 2010
[7]. D.,. Mane , D., Robescu, and A., Presura.
“Sustainable Energy In Waste Water Treatment
Plants”. ISSN 1454-2358. 2012.
[8] S., Stillwell, D.C., Hoppock, Michael E., Webber
"Energy Recovery from Wastewater Treatment
Plants in the United States: A Case Study of the
Energy-Water Nexus," Ph.D. Sustainability, 2, 945962; doi:10.3390/su2040945, 2010.
[9] R., Volpe., “Techniques for Collision Prevention,
Impact Stability, and Force Control by Space
Manipulators,” Teleoperation and Robotics in Space,
edited by S. B. Skaar and C. F. Ruoff, Progress in
Astronautics and Aeronautics, AIAA, Washington, DC,
pp. 175–212, 1994.
[10]. P.A., Vanrolleghem..“ Model Based Control Of
Waste Water Treatment Plants. Department
Applied Mathematics”, Biometrics and Process
Control University of Gent, Coupure Links 653, B9000 Gent, Belgium.
11