Biogas Systems
Dr David Fulford CEnv MEI
www.kingdombio.com
davidf@kingdombio.com
• Dr David Fulford
• Running a Biogas Programme:
A Handbook (1988)
• Practical Action Publishing
• Available from:
• http://developmentbookshop.
com or
• Amazon
• Small-scale, domestic biogas
programmes (2014)
• available by end of October
Biogas (anaerobic) digestion
• Biogas digestion: takes place in sewage farms, landfill gas sites
and specialist biogas plants.
• Microbes naturally occur in marshes and animals' guts,
especially ruminants e.g. cattle.
• Need special conditions:



no oxygen (anaerobic)
no light
moderate temperature: (20°C to 55°C)
• Biogas process:
• population of symbiotic microbes in a suitable environment:
• secrete enzymes to break down food into soluble chemicals, give
off energy.
• Symbiotic, since later microbes use waste products of earlier
organisms (bacteria and archaea ).
Biogas process
Feed Material
Carbohydrates Fats
Proteins
HYDROLYSIS
Soluble organic matter
Sugars, Alcohols Fatty Acids
Amino Acids
Indigestible
matter
ACIDOGENESIS
Volatile Fatty Acids (VFAs)
Ammonia
ACETAGENESIS
Acetate
Formate, H2, CO2
METHANOGENESIS
Biogas: CH4, CO2
Slurry
Operating parameters (1)
• Moisture content:
• Optimum: 10% solids and 90% water (usually mix with water)
• < 4%, get separation, solids sink to bottom
• >12%, slurry will not flow, acts like a gel
• Solids digestion, produces biogas; need to push slurry around
• pH, acidity:
• Optimum pH: 6.5 to 7.5 (neutral)
• Anaerobic digestion – self buffering, carbon dioxide reacts with
carbonates to make bicarbonates
• If biogas process fails, methanogenesis fails first, so plant goes
“acid” and pH drops.
• Volatile fatty acids smell bad (acetic acid = vinegar, butyric acid =
rancid butter)
Operating parameters (2)
• Temperature:
• Three temperature ranges
• Cryophyllic – 10 to 20°C – biogas does not work well
• Mesophyllic – 30 to 40°C – optimum temperature
• Thermophyllic – 45 to 60°C – better performance, unstable
• Solid properties:
• Small particles allow microbes to reach food
• Grind up fibres, allow them to stay in suspension
• Toxins:
• Antibiotics, disinfectants and pesticides kill microbes.
• Chlorine compounds are very bad.
• Organic soaps are good, can be digested
Modes of operation
• Batch
The feed material placed in a container and allowed to decompose
• Material can be loaded and unloaded from containers
• Gas increases sharply and then decreases slowly
• Usually need several plants to get a steady gas production
• Start each one after a few days
• Continuous:
Feed material pumped in steadily
• Get very steady gas production
• Complex to run, need liquid slurry
• Semi-continuous:
Feed material added each day, mixed into a liquid slurry
• Get a fairly steady gas production (small daily ripples)
Digester Functions
• What does a digester do?
– Contains slurry for a suitable time
– Keeps out air and light
– Maintains a suitable temperature (35°C or 55°C)
– Collects gas in gas-tight container and stores it until needed.
• Sum up: keep bacteria happy and producing gas
• Design parameters
– Strength
- Leak tightness
– Cost
- Built of local materials
– Ease of use
- Ease of maintenance
– Ease of insulation and heating
- Reliability
Digester Scales
• Family sized digesters
– Internal volume about 4 m3
– Dung from 4 cattle or 8 pigs
– Gas used for cooking meals
• Commerical digesters
– Internal volume from 20 m3 to 10,000 m3
– Dung from cattle herds, food processing residues or energy crops
– Gas used to run engines to generate electrcity
• Sewage digesters
– Internal volume from 60 m3 to 10,000 m3
– Sewage from institution or local community
– Gas used to run engines to generate electricity
Digester Applications (1)
• Family sized digesters
– used widely in China (about 40 million), India (about 4 million) and
Nepal (about 1/4 million), Rest of Asia (about 1/4 million) (2011 figures)
– low cost, although programmes needed subsidies
– mainly in tropical areas, so no heating used
– mainly use animal dung (cattle or pig)
– mainly used for cooking food, some for gas lights
• Sewage digesters
– used in UK and Europe to process urban sewage
– often gas used in engines to generate electricity
– heat from CHP (combined heat and power) to heat digesters
– projects in Rwanda and Ghana for institutions with gas for cooking
Digester Applications (2)
• Animal dung, food waste and energy crop digesters
• Large-scale systems in Germany and rest of Europe
• German statistics:
Year
No. plants installed
No. used for biomethane
Electricity (MW) generated
2010
2011
2012
2013
5,905
7,320
7,589
7,874
45
80
95
112
2,291
2,993
3,179
3,364
• German Feed-in-Tarrif different for waste and crops
• Some systems in UK: 148 in September 2014, but FIT same for
waste and crops (excludes 146 sewage plants in UK).
Digester Applications (3)
• Small-scale food waste and crop wastes in India
– Domestic scale - food waste saves 25% LPG for cooking
– Institutional scale - food waste + sewage saves 50% LPG
– Local Authority scale - gives electricity for lighting
used for market waste
• Main use of biogas is for household cooking:
– domestic, rural, with animal dung as feed material
• Brief history: project started in Nepal (DCS of UMN)
– local management (ADB/N - funded by UNDP)
– SNV supplied volunteers, took over project as BSP/N
– SNV project extended to Vietnam, Bangladesh, Cambodia, Laos
– now in many other countries.
Benefits (1)
• Clean gaseous cooking fuel
• No smoke
• Instant availability
• Does not need constant attention
• Reduced danger of burns
• Resource (dung)
available from animal
sheds
• No need to walk to
collect firewood
• No insects from wood
store (cockroaches)
Benefits (2)
• Cooking pots easy to clean (no soot)
• Saving of time (3 hours a day)
• Saving of firewood (2,000 kg a year)
• Reduced deforestation (1,000 biogas plants
saves 33.8 ha forest from clear felling- WWF)
• Much reduced smell
from the animal sheds
(in Vietnam, pig sties
are close to the house)
Benefits (3)
• Biogas can be used for lights
• Reduced smell from kerosene lamps
• Savings of 32 litres kerosene a year
• Reduced risk of house fires
• Saving of carbon (4,900 kg a year)
• Since gas available in
the morning, children
get cooked breakfast
before school.
Benefits (4)
• A latrine can be attached
• Improved sanitation
• Reduced transfer of pathogens (especially if
slurry is properly processed)
• Reduced risk to women
(who go out at dawn or
dusk to use the fields)
• Reduce incidence of
snake bites
Benefits (5)
• Slurry is a good quality compost (better than
raw dung)
• Liquid slurry should be absorbed in dry
biomass and composted for 1 month
• Compost even better if
use vermi-culture
• Growers prepared to
pay cash for vermicompost
Floating Drum
Designs
– Used in India,
developed by KVIC
– Brick lined pit in
ground
– Floating Drum made
from steel
Small Digester Designs (2)
• Fixed dome designs
– Developed in China, but used widely
– Masonry lined pit covered with masonry dome
– Uses displacement principle to store gas - push slurry into
reservoir
Displacement Principle
As gas collects under dome, slurry is pushed into reservoir
As gas is used, the slurry flows back into plant.
Small Digester Designs (3)
• Deenbandhu fixed dome design
– Developed in China, but used widely in India and also Africa
– Made from bricks placed in rings to form hemisphere
– Cheap to build, but needs skilled masons
Gas Burner Design
Throat
Burner ports
Jet
Air
holes
Mixing
tube
Burner
manifold
• Use of gas in burners
–
–
–
–
–
–
Biogas cannot be used in burners for natural gas or LPG
Need to design burners for properties of biogas
Gas enters via jet, size controls flow
Air is entrained and mixed with gas (primary air)
Mix directed to burner ports, burns in air (secondary air)
Stochiometric mix: air to completely burn biogas 5.53:1
Food Waste Digestion (1)
• Food waste contains more energy than animal dung
– Animal has used energy for itself
 BUT animal has pre-processed biomass:
– so dung contains ground up biomass
– and contains useful microbes
 Food waste need pre-processing
– may need to be chopped up
– needs to be mixed with microbes
 Single stage system
– need fine particles that remain in suspension
– all food waste flows through digester
 Two stage system
– two separate containers
– allows indigestible material to be removed
Food Waste Digestion (2)
• Examples of food waste digesters
 Nisargruna process (BARC) in Mumbai
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–
–
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waste collected from households
ground up in chopper
two stage digester, but no separation
first stage uses heated aerobic process
 Biotech Ltd in Kerala, South India
– first stage uses leach bed, recycled liquor
– waste allows to dry out and dug out as compost
– no smell, VFAs leached out to make biogas
 Bioplex in UK (now Longstock Energy Ltd)
– uses a flooded bed first stage with added liquor
– liquor drained out into main digester and solids removed
Nisargruna System
Sewage Systems
• Examples of sewage digesters
 KIST project in Rwanda (Kigali Institute of Science
and Technology)
– Sewage from jails, with 10,000 people in jails
designed for 1,000
– Replaced 50% of firewood used for cooking
– Smell free compost used to grow food
 BTAL in Ghana (Biogas Technology Africa Ltd)
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–
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Sewage from hospitals, schools, colleges
Local sewage dumped in rivers and sea
Replaces ineffective sewage treatment plants
E.g. aerobic system bubbled air through lagoons
Aerobic digestion in Ghana - air blown lagoons - failed
(German system)
Further information
Further information on biogas can be found at:
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•
www.ashden.org - from where the films came
www.kingdombio.co.uk - my own work
foundationskgsangha.org - charity linked to Indian NGO
www.iea-biogas.net - IEA Task 37 - mainly larger systems
biogas.wikispaces.com - Discussion group, information
pages organised by Paul Harris, Adelaide University
• www.snvworld.org/en/sectors/renewableenergy/publications - SNV (Netherlands Development
Organisation) has projects in many countries
• www.biogasonline.com – work in Ghana (BTAL)