Biogas

Biogas
• Biogas is produced from anaerobic digestion of
organic wastes
• Biogas is a mixture of gases contains methane,
carbon dioxide and hydrogen sulphide
– methane (CH4): 40-70 vol.%
– carbon dioxide (CO2): 30-60 vol.%
– other gases: 1-5 vol.% including
• hydrogen (H2): 0-1 vol.%
• hydrogen sulfide (H2S): 0-3 vol.%
Bio…
 Factors Affecting anaerobic digestion
• Anaerobic condition
• Temperature
• Feeding
• Carbon to nitrogen ration(C/N)
• pH
• Retention time
• Inhibitory factors
 Factors….
• Anaerobic condition
Many groups of bacteria involve but the most
important bacteria are methane producing
bacteria => very sensitive to oxygen
If oxygen is available in the digester either
they will die or grows slowly
 <0.08mg/l of dissolved oxygen will inhibit
their growth completely
Air tight digester is required
 Factors….
Substrate Temperature
• The ideal temperature for biogas generation is about
35oC.
• Three temperature ranges for methane production
1. Thermophilic (45-60oC)
• It will operated at high loading rate=> high gas
production will be obtained
• Heating system and insulation is necessary (needed)
• Cost of operation and construction is high
 Factors….
2. Mesophilic (10-45oC)
• Runs at medium gas production compared
with thermophilic
• The cost is lower
3. Psychrophilic
Certain bacteria conduct anaerobic digestion
below 10oC
Does the temperature of ambient has an impact on
the biogas generation?
 Factors….
Feeding
Common feed materials include animal wastes, crop-
residues and urban wastes
1. Feeding quantity
Determined by the size of the plant and retention time
2. Feeding quality
Feeding
material

Solid matter Water

Non volatiles
Volatiles VS-
(FS=fixed
volatile solids
soilds)
 Factors…

Fresh • For easy mixing and

cattle dung handling 8-10% TS in
the feeding is
20% TS 80% water
recommended
• To make the fresh
cattle dung from 8-
70%VS 30%FS 10% TS what will be
the amount of water
or urine added?
 Factors…
Type of feeding material Rate of gas production in Daily dung production per
liter (cum) producer (kg)

Cattle dung (cow/ox) 35-40(0.035-0.040) 10-15

Buffalo 35-40(0.035-0.040) 15-20

Pig manure 40-60(0.04-0.06) 2-4

Poultry dropping 55-70(0.055-0.07) 0.02-0.03

Human Excreta 45-50(0.04-0.05) 0.18-0.34

If kitchen waste is used as a feedstock and the digester is well designed, the
biogas yield will be 0.3m3/kg of waste.
 Factors…
Carbon to nitrogen ration(C/N)
• Organic matter will contain carbon, hydrogen,
nitrogen, phospurs, sulfur
• C/N ratio is an important index to calculate the
capacity of materials to decompose
• Biogas microbes needs 25 to 30 times higher
carbon than nitrogen
• Optimal C/N ratio for anaerobic degradation
depends on the substrate used but a value
between 20 to 30 is recommended
 Factors…
pH
• The methane-producing bacteria live best under
neutral to slightly alkaline conditions
• Once the process of fermentation has stabilized
under anaerobic conditions, the pH will normally
take on a value of between 7 and 8.5.
• A digester containing a high volatile-acid
concentration requires a somewhat higher-than-
normal pH value.
• If the pH value drops below 6.2, the medium will
have a toxic effect on the methanogenic bacteria.
 Factors…
Retention time
• It is the time required by a given amount of
organic material to produce 80 to 85% of the
total gas (the time spent by the feed inside the
digester before it is completely digested)
• RT determined by the temperature of the slurry
and the digester volume
• RT period differs from place to place
• for animal manure around a month will be
enough but for plant based materials the time
will be longer
 Factors…
Manure Gas production (liters) as per RT in days

25 days RT 30 days RT 35 days RT 50 days RT

Pig 58 63 68 77

Cow 30 34 37 43

Buffalo 30 34 37 43

Horse 45 51 56 65

Chicken 60 65 69 78

Human excreta 50 55 59 65
 Factors…
Limit concentrations (mg/l) for
• Inhibitory Factors various inhibitors
• The presence of heavy
metals, antibiotics
(Bacitracin, Flavomycin,
Lasalocid, Monensin,
Spiramycin, etc.) and
detergents used in
livestock husbandry can
have an inhibitory effect
on the process of bio-
methanation.
 Biogas digester
• The core of a biogas plant is the digester - an air proof
reactor tank, where the decomposition of feedstock takes
place, in absence of oxygen, and where biogas is produced
• Common characteristics of all digesters:
– air proof
– system of feedstock feed-in
– systems of biogas and digestate output
– Heating system (when installed in cold climate)
• There are a various types of biogas digesters, operating
around the world
• Digesters can be made of concrete, steel, brick or plastic,
shaped like silos, troughs, basins or ponds, and they may be
placed underground or on the surface
Biogas dige….
 Floating drum plants
• The gas-holder floats either
directly in the fermenting slurry
or in a separate water jacket.
• The drum in which the biogas
collects has an internal and/or
external guide frame that
provides stability and keeps the
drum upright.
• If biogas is produced, the drum
moves up, if gas is consumed, the
gasholder sinks back.
 Biogas dige….
• From the point of view of feedstock
input and output, there are two main
basic digester types:
–Batch and
–Continuous
• Some times we will have a semi batch
type of digesters
 Sizing of a biogas digester
• Size of the biogas plant depends on quantity,
quality, kind of biomass and digesting
temperature
Vd = Sd*RT
• Where Vd (m3) = volume of the digester
Sd (m3/d)= daily substrate input
RT (d) = Retention time
Substrate input = biomass (B) + Water(W)
 Sizing of….

Daily gas production (G)

It can be Calculated based on
1. specific biogas yield (Gy(m3/d.kg)) and
Volatile solid content (Vs(kg))
G = Vs*Gy
2. The weight of moist biomass (B)
G = B*Gy
 Sizing….
• Specific gas production (Gp)
Gp = G/Vd (m3/day/m3)
• Digester loading Ld
A) Based on total solid

B) Based on daily volatile solid

 Sizing…..
• The gas holder must be designed to
– Cover the peak consumption rate gcmax (~Vg1)
– Hold the gas produced during the longest zero
consumption period tzmax (~Vg2)
Vg1= gcmax*tcmax= Vcmax
Vg2=tzmax*Gh=Vcmax
Where
gcmax = maximum hourly gas consumption [m3/h]
tcmax = time of maximum consumption [h]
vcmax = maximum gas consumption [m3]
Gh = hourly gas production [m3/h] = G ÷ 24 h/d
tzmax = maximum zero-consumption time [h]
 Sizing…
• The larger Vg-value (Vg1 or Vg2) determines
the size of the gasholder.
• A safety margin of 10- 20% should be added:
• Vg = 1.15 (±0.5) × max(Vg1,Vg2)
 Exercise
• Assume you have a biogas plant at your home
and the peak consumption rate is 40m3/h for
about 4 hours, the longest zero consumption
time is 5 hour, and hourly gas production rate
is 45 m3. what will be the size of the gas
holder?
 Reading Assignment
• Landfill
Questions?
Thank You!