Post on 11-Feb-2022
Converting Waste Agricultural Biomass
into Energy Source
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By:
Prof. Dr. Rasool Bux Mahar
Institute of Environmental Engineering & Management Mehran University of Engineering & Technology
Jamshoro, Pakistan
United Nations Environmental Programme International Environmental Technology Centre
Osaka/Shiga, Japan
2
PREFACE
Rapid increase in demand and consumption of fossil fuels and its consequent
impact on climate change and environment has put greater emphasis on development
of alternative and renewable sources of energy. Waste biomass, as a renewable energy
source, presents a viable solution for meeting our energy demands. It addresses the
climate change issues as well as reduces our dependence on fossil fuels. In developing
countries this could be developed as a versatile source of energy for domestic as well
as industrial/commercial purposes.
In this report district Sanghar, which is one of the largest districts of province
Sindh, Pakistan was selected for “converting waste agriculture biomass into energy
into energy” in which one of the objective is to develop pilot project of environment
sound technology(EST) and train the local government/ stake holders/partners to
implement the technology. In this regard, Pilot Scale Biogas plant for Waste
Agriculture Biomass at Sanghar Sugar Mill is to be developed. The economic and
environmental feasibility of the pilot project is presented in this report.
This biogas plant will be the first plant in Pakistan which is to be operated by
using waste agriculture biomass. Biogas generated is to be utilized for heating,
cooking and lightning purpose. Existing WAB disposal methods are open dumping
and burning into the environment. At dumping side, organic matters are being
fermented and producing lot of greenhouse gases (GHG) to the environment and at
open burning side flue gases are being emitted into the environment. The said pilot
project is environmentally and economically viable in which major portion of GHG
methane is to be captured by fermentation of organic matters in digester to produce
the heat energy. The lesson learned from this project various similar type of the
biogas plants to be replicated in remote areas of the country with cooperation of
district/ local government. This project would reduce the consumption fossil fuels and
resultantly would reduce emissions of greenhouse gases.
3
TABLE OF CONTENTS
Description Page
Preface 2
1 INTRODUCTION TO THE PROJECT 4
2 PILOT SCALE BIOGAS PLANT FOR WAB 5
3 WORKING PROCESS OF BIOGAS PLANT/ANAEROBIC
DIGESTER (AD)
5
4 FEEDSTOCK 6
5 DIGESTATE 6
6 UTILIZATION OF BIOGAS 6
7 DESIGN OF BIOGAS PLANT 7
7.1 Temperature 7
7.2 Hydraulic Retention Time 8
7.3 Digester 8
7.4 Feed tank 8
7.5 Mixing of Feedstock 8
7.6 Feedstock Heating 9
7.7 Gas Storage 9
7.8 Digestate Tanks 9
7.9 Digestate Storage Tanks 9
8 COST ESTIMATE OF BIOGAS PLANT 10
8.1 Capital cost of the pilot project 10
8.2 Operational cost of the pilot project 10
9 INCOME FROM THE PILOT PROJECT 11
10 ECONOMIC FEASIBILITY OF THE PROJECT 12
11 ENVIRONMENTAL FEASIBILITY OF PROJECT 13
12 PROJECT IMPLEMENTATION STRATEGY 13
Annexure – A: DETAIL DESIGN AND DRAWINGS 14
4
1 INTRODUCTION TO THE PROJECT
The agricultural residues are referred as the Waste Agricultural Biomass,
which may be assorted as field residues that is the matter leftover in an agricultural
field after the crop has been reaped e.g. leaves, straw, stalks, roots etc. and process
residues that is the matter leftover after the processing of the crop at the mills or
factories into a valuable resource e.g. husks, sugar cane fiber (bagasse), seeds,
groundnut shells, maize cobs etc.
The waste agricultural biomass is the valuable by product of the crops
cultivate. Worldwide a huge quantity of the WAB is generated, out of which a
considerable quantity is being either wasted without any use or its use is improper.
Regarding this UNEP has a project entitled “Converting Waste Agricultural Biomass
into Fuel/Resource” in Pakistan.
The objective of the overall project is to assist the government in identification
and implementation of environmentally sound technology (EST) for converting waste
agricultural biomass into energy/material source.
The project will build the local capacity to identify and implement EST for
waste agricultural biomass recycling and assess their potential for resource
conservation and GHG emissions reduction as well as for their feasibility with respect
to local socio-economic and environmental characteristics. The operability and
benefits of the selected ESTs will be demonstrated through pilot project.
This project is in direct support of Bali Strategic Plan for Capacity Building
and Technology Support. It is aimed that local capacity will be strengthened in data
collection and analysis to develop baseline scenarios for cities/countries on
quantification and characterization of waste agricultural biomass as well as on
prevailing management systems including regulations/policies. It is also aimed that
local capacity is built for identification of appropriate technologies and assessment of
their potential for resource conservation and GHG emissions reduction. It is also
aimed that local capacity will be strengthened on procurement and implementation of
the EST with operation and maintenance skills.
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2 PILOT SCALE BIOGAS PLANT FOR WAB
The pilot scale Biogas plant is to be built at Sanghar Sugar Mill with
cooperation of Sanghar Sugar Mill management and Local Government. The location
of the Biogas plant and collaboration partners were decided selected in stake holders’
concerns workshop held in District Nazim Secretariat under the chairmanship of the
District Coordinator Officer (DCO) Sanghar.
Biogas is to be produced by anaerobic digestion of Waste Agriculture Biomass
which includes baggasse, animal dung, banana plant waste and other crop residues.
Anaerobic Digestion is the biological treatment of biodegradable organic waste in the
absence of oxygen, utilizing microbial activity to break down the waste in a controlled
environment. Anaerobic digestion results in the generation of:
1. BIOGAS, which is rich in methane and can be used to generate heat and/or
electricity;
2. FIBER, (or digestate) which is nutrient rich and can potentially be used as a
soil conditioner; and
3. LIQUOR, which can potentially be used as a liquid fertilizer
Anaerobic digesters produce favorable conditions that encourage the natural
breakdown of organic matter by bacteria in the absence of air. Anaerobic digestion
(AD) provides an effective method for turning residues from livestock farming, Crops
forming, organic fraction of municipal solid waste and food processing industries
waste into the useful products as stated above.
3 WORKING PROCESS OF BIOGAS PLANT/ANAEROBIC DIGESTE R
(AD)
The digestion process takes place in a sealed airtight container (the digester)
which creates the ideal conditions for the bacteria to ferment the organic material in
oxygen-free conditions. The digestion tank needs to be warmed and mixed thoroughly
to create the ideal conditions for the bacteria to convert organic matter into biogas
(mixture of carbon dioxide, methane and small amounts of other gases).
During this process 30 - 60% of the digestible solids are converted into biogas.
This gas must be burned, and can be used to generate heat or electricity or both. It can
be burned in a conventional gas boiler and used as heat for nearby buildings including
farmhouses, and to heat the digester. It can be used to power associated machinery or
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vehicles. Alternatively, it can be burned into gas engine to generate electricity. If
generating electricity, it is usual to use a more efficient combined heat and power
(CHP) system, where heat can be removed in the first instance to maintain the
digester temperature, and any surplus energy can be used for other purposes.
4 FEEDSTOCK
A fresh feedstock is to be added to the system continuously, which produce
biogas and collected into the storage tank. For this project, during operation period of
sugar mill baggase residue (size 1-15 mm) is co-digested with animal dung and during
closure period of sugar mill banana plant residue, cotton stalk and other free available
WAB is to be utilized. The proportion of WAB and animal is to be kept 75% and 25%
respectively. Feedstock is to be hydrolyzed in the hydrolyzing tank then is to be feed
in the digester from inlet daily and same volume is to be drained out from the out let.
5 DIGESTATE
The residual digestate is to be stored and then applied to the land at an
appropriate time without further treatment, or it can be separated to produce fiber and
liquor. The fiber is to be used as a soil conditioner or composted prior to use. The
liquor contains a range of nutrients and can be used as a liquid fertilizer which can be
used in agriculture field as a part of a crop nutrient management plan.
AD products can, therefore, help farmers reduce their requirement for non-
renewable forms of energy such as fossil fuels, and the digestate, if correctly used,
can reduce demand for synthetic fertilizers and other soil conditioners which may be
manufactured using less sustainable methods.
6 UTILIZATION OF BIOGAS
The biogas generated is to be utilized for cogeneration with baggase for
heating the boiler of Sugar Mill. Presently hard wood is being used as cogeneration
with baggase. There is no natural gas supply line in the vicinity of Sugar Mill. In off
season of Sugar Mill the biogas is to be utilized for cooking of food for workers and
biogas generator could be run for generation of electricity for sugar mill office during
load shading. Usually breakdown of electricity is here about 6-8 hours per day.
7 DESIGN OF BIOGAS PLANT
7
The following operational parameters of biogas plant are designed and
mentioned in the Table 1 and detailed drawing is provided in annexure A and
description of the designed parameters is given below:
7.1 Temperature
The proposed biogas/anaerobic digestion plant will work of mesophilic a
condition that is within the temperature range of 37 ºC to 40 ºC.
Table 1 Design parameters of Biogas plant
Parameters Design Values
Hydraulic Retention Time (HRT) 20 days
Temperature 37o -40 o C
Total Quantity of WAB to be added 400 kg/ day
Total quantity of the water to be added 430 kg/ day
Volume of Digester (R.C.C) 20.75 m3
Biogas yield per day 50 m3
Use of Biogas Heating of Boiler (co-combustion with baggase)
Diameter of digester 10 ft
Height of Digester 9 ft
Hydrolysis Tank (0.8 m3) (w × l × h) 4’ × 4’ × 3’
Lower Digestate Tank (Bricks) (w × l × h) 3’ × 4’ × 2’
Solid fertilizer 02 tanks (Bricks) (w × l × h) 3’ × 5’ × 5.5’
Liquid fertilizer tanks 02 Bricks (w × l × h) 3’ × 5’ × 5.5’
Gas Storage Tank (mild steel) length = 8’ & diameter = 4’
Gas pile line 1 ” G.I.
Stirring by Mechanical Mixer Horizontal (motorized)
Inlet & Outlet pipes 6 ” (stainless steel)
Inlet valve 6 ” (stainless steel)
Land required 2000 sqm (0.5 acre)
8
7.2 Hydraulic Retention Time (HRT)
The hydraulic retention time is to be kept as 20 days. If the HRT is too large
then bigger size of the digester is to be needed and on other hand if HRT is too small
then prior to complete digestion of organic matters will come out from the digester.
For completely mixed anaerobic digester HRT is required about 6-10 days. However,
in case of this project HRT is to be kept about 20 days because digester is to be
operated as a partially mixed reactor.
The bio-digester is single stage and will be made from concrete. The volume
of the digester is 20.75 m3, will produce biogas of 50 m3 per day. The detail of the
other components of the plant is described in following subsections.
7.3 Digester
It is the cylindrical tank made up of reinforced concrete. The inner diameter of
the cylinder is 10 feet; where as its effective height is 9 feet. Above the 9 feet height
there is a dome, also made up of reinforced concrete. An opening is made in the
center of the dome having the diameter of 2 feet. There is a one inlet at the height of
one foot above the bottom and two out lets, the first is at 2 feet above the bottom and
the second is at the opposite side to the first outlet and is at the height of 8 feet from
the bottom.
7.4 Feed tank
A single feed tank is provided into the plant. The feed tank is to be installed on
the platform at the height of 7 feet from the ground level. The access to the platform is
through the stair. The stairs and the platform are to be made from reinforced concrete.
The volume of the feed tank is designed as 0.8 m3. The frequency of the feeding is
daily. The feedstock consists of 100 kg of cattle dung, 300 kg of WAB and about 430
kg of water.
7.5 Mixing of Feedstock
The designed anaerobic digestion plant works on the co-digestion of cattle
dung and waste agricultural biomass. These are to be mixed with the water. I order to
get the homogenized mixture of the feed stock a mechanical mixer will be installed,
which will be driven by the electric motor.
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Another mechanical mixer is for the mixing of the feedstock within the
digester to avoid scum formation and increasing efficiency of the overall process of
digestion. This mechanical mixer is paddle type and is motorized.
7.6 Feedstock Heating
In case of cold climate the temperature of the digester is to be kept at 37 ºC in
order to get constant production of biogas. If the temperature decreases to the 25 ºC
then the process becomes psycrophilic and the biogas production decreases. One of
the simplest methods is to increase the temperature of the digester in the cold climate
to preheat the feedstock. For this at the bottom of the feed tank a biogas stove is to be
installed. It will consume fraction of the produced biogas for heating the feedstock.
The digester is exposed to the sun, thus solar radiations will directly hit the
digester and causes the increase in the temperature of the digester within the range of
mesophilic condition.
7.7 Gas Storage
In addition to the dome provided into the digester as a biogas storage space, a
separate gas storage tank is to also be provided. This tank is made up of mild steel.
The shape of the tank is cylindrical and will be laid down on the ground horizontally.
The diameter of the tank is 4 feet and the length is 8 feet. The total volume of the tank
is 100 ft3. The inlet and the exit from the gas storage tank are from the top, which
insures the moisture trap. The trapped moisture from the gas storage tank can be
exhausted from the valve located below the tank. The pipeline and fitting used for the
biogas transfer is made up of galvanized iron and having the size of 1 inch diameter.
7.8 Digestate Tanks
There are two digestate tanks in the plant. The main digestate tank is equal in
size to the feed tank that is 0.8 m3. As per design parameters the amount of the
feedstock entered into the digester will be equal to the amount of digestate drawn
from it. The second digestate tank is located at the bottom of the digester.
7.9 Digestate Storage Tanks
There are four storage tanks, two for the solid digestate and two for the liquid
fertilizer. One set of the tanks will remain in operation at once, while the other
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remains with the digestate filled for its settling. The digestate will transfer from
digestate tank to storage tank through 3 inches galvanized iron pipeline along with the
valve.
8. COST ESTIMATE OF BIOGAS PLANT
The cost of the biogas plant is splitted into two parts i.e. capital cost and
operational cost.
8.1 Capital cost of the pilot project
The capital cost of the project is estimated on the basis of quantity of material
and equipments. Major cost is required to build the Bio-digester which is to be built of
Reinforced Cement Concrete (RCC) along with inlet and outlet ports and for
hydrolyzing Tank, gas holding Tank, Mixers with motors and gas valves, liquid and
solid bio-fertilizer storage reservoirs. The rough estimated cost is given in the Table 1.
The land required for pilot project is provided by the Sugar Mill Management.
8.2 Operational cost of the pilot project
The operational cost of pilot project is estimated on the basis of operational
requirements for the Biogas plant which includes feedstock, electricity consumption
and salary of the operator. The details of the operational cost are given in the Table 2.
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Table 2 Cost estimates for Biogas Plant
COST OF THE PROJECT
Operational Cost
Description Rate/ unit Quantity
(tons/year)
Total cost in
Rs.
Baggase Residue+ Banana waste
+ other crop residue Rs.1000 / ton 108 108000
Animal Dung Rs.500 / ton 36 18000
Auxiliary electricity Rs.10 /Kw 2880 28800
Operator salary Rs.10000/ month 12 120000
Total operational cost Rs/ year 274800
Capital cost
Civil work for digester, Pumps, Motors, Mixers, Physical
Infrastructure, including Design and Implementation cost in Rs.
2000000
Note: 1 USD=Rs. 86
9. INCOME FROM THE PILOT PROJECT
Three types of products are to be produced from pilot project are given below:
1. Biogas
2. Liquid fertilizer
3. Solid fertilizer
The quantity of biogas to be produced is estimated as per literature and
composition of feed stock to be added to the digester. The quantity of methane is
estimated by assuming the concentration of methane in biogas is 60%. The quantity of
biogas and fertilizer is to be obtained from the project are given in the Table 3. The
rate of uncompressed methane is assumed as Rs.40/kg. However, in the market rate of
compressed natural gas (CNG) is Rs.53/kg. The rate of liquid fertilizer and solid
fertilizer is assumed as Rs.1.5 per kg and Rs.4 per kg respectively. Income generated
from the biogas and fertilizer is mentioned in the Table.4.
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Table 3 Quantity of biogas and fertilizer produced from Biogas plant
PRODUCTION OF BIOGAS PLANT
Products of the project Units Quantity
Biogas generation m3/ month 1680
Equivalent to Natural gas m3/ month 1008
Methane equivalent in kg/day 8588
Production of liquid fertilizer kg/day 200
Production of solid fertilizer kg/day 150
Production of Liquid fertilizer kg/year 72000
Production of solid fertilizer kg/year 54000
Table 4 Income generated from the plot project
INCOME FROM THE PROJECT
Total income from the project Rate/ unit
Quantity
in kg/
year
Total in Rs.
Income from Liquid fertilizer in
Rs/year Rs.1.5/kg 72000 108000
Income from Solid fertilizer in Rs
/year Rs.4/kg 54000 216000
Income from Methane in Rs per year Rs.40/Kg 8588 343526
Total income Rs. Per year 667526
Operational cost per year 274800
Net saving Rs. per year 392726
Pay back period years 5
10. ECONOMIC FEASIBILITY OF THE PROJECT
From the analysis of capital cost and operational cost of biogas plant and
income generated from the project, it is observed that project is economically feasible.
The project life of the project is assumed about 20 years and in first 05 years project
cost would be recovered/ pay back as given in the Table 4.
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11. ENVIRONMENTAL FEASIBILITY OF PROJECT
Current WAB management methods are open dumping and burning into the
environment. At dumping side, organic matters are being fermented and producing lot
of greenhouse gases (GHG) to the environment and at open burning side flue gases
are being emitted into the environment. The said pilot project is environmentally and
economically viable in which major portion of GHG methane is to be captured by
fermentation of organic matters in digester to produce the heat energy.
Rapid increase in demand and consumption of fossil fuels in Pakistan and its
consequent impact on climate change and environment has put greater emphasis on
development of alternative and renewable sources of energy. Waste biomass, as a
renewable energy source, presents a viable solution for meeting our energy demands.
It addresses the climate change issues as well as reduces our dependence on fossil
fuels. In Pakistan, this pilot project will be developed as a versatile source of energy
for domestic as well as industrial/commercial purposes.
12. PROJECT IMPLEMENTATION STRATEGY
The project is to be implemented with cooperation of Sanghar Sugar Mill
management. The Sanghar Sugar Mill authority is agreed to own and contribute in the
pilot project of Biogas Plant in form of money and land (Provide Rs. 400000.00 and
provide required land) for it. They are also agreed to operate and maintain the plant
minimum for 10 years. Mehran University of Engineering and Technology (MUET)
Jamshoro will provide all technical guidance from execution to operation of the
project. The products obtained from the pilot project will be utilized by the Sugar
Mill. The Construction of Bio-digester along with accessories and procurement of
equipment are to be carried out through the inviting tenders from contractor /vendors.
The work is to be awarded after proper evaluation of tenders to lowest vendor/
contractor. The break up of the funds is given as below:
Estimated cost of plant = Rs. 2000000/-(23256 USD)
Amount to be paid the by partner = Rs. 400000/- (4705 USD)
Amount to be paid by UNEP(1USD=Rs.86) = Rs. 1720000/-(20000 USD)
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Annexure – A: DETAIL DESIGN AND DRAWINGS
5'
4'
5'-6"3'
4'
6"
Digesate Tank
Digesate Exit
2'
10'-6"
4'
3'
G. Level
Digesate Tank
Ø10'
Ø11'-6"
Ø16'
2'
5'
Fig. A: Plan and section of Bio-digester showing the inlet and out let arrangement
Digestate Storage Tanks
Digester
Digestate Main Exit
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G. Level
3'-6"
4'
Gas Storage
Feed Tank
Gas to Consumer
4'
8'4'
2'
9'
Ø16'
Ø11'-6"
Ø10'4'
3'
4'
5'
Stairs
3'7'
3'-6"
3'
Fig. B: Plan and section of Bio-digester showing feeding and gas tanks arrangement
Inlet
16
9'
1'
# 5 @ 6" c/c
#4 @ 6" c/c
Ø10'
Ø11'-6"
Ø16'
Main Hole
6", 1:2:4
#3 @9" c/c
#3@9"c/c
#4 @ 6" c/c
Figure C: Structural design drawing of AD
17
Figure D: The Digester Detail Drawing
Ø10
'
Ø11
'-6"
Ø16
'
3'-6
" S
q
3' 9'Ø10
'
8'-9
"
2' u
nder
grou
nd
7'-6
"
2'-6
"
2'-9
"
Mai
n D
iges
ate
Exi
t
Dig
esat
e E
xit
Dig
esat
e S
ampl
e P
oint
Mai
n H
OLE
Dig
esat
e In
let
AA
SE
CT
ION
A-A
9"
18
Ø1"
Ø1'Ø6"
Ø1'
1" 2"
9" 7"
Figure E: Digester Exit Pipe
6" 1'
Ø6"
Ø1'
Ø1" 1" 2"
115°
7"9"
Figure F: Digester Inlet pipe along with bent
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3" 9"
Ø1"
Ø6"
6" 2" 10"1"
Ø7"
Figure G: Digester Sampling point exit pipe
GAS INLET
GAS EXIT
4'
8'
Figure H: Bio-gas storage tank