green earth design template · triple benefit of biogas • it is important to view the biogas...
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BioGas Basics By David Cilliers
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What is BioGas?
• This gas production occurs naturally through decomposition in the absence of oxygen (Anaerobic).
• A methane rich gas. • Commercially generated from waste.
– In some countries crops are grown for the specific purpose of producing biogas.
• Used to replace fossil fuels.
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What is BioGas?
• It is also called Marsh or Swamp gas. – Naturally occurs in these places.
• Most organic waste left to decompose will produce BioGas.
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BioGas description
• BioGas is not a single gas. – Methane 45-65% – Carbon Dioxide 35-55% – Other gasses less than 5%
• Water vapour • Hydrogen Sulphide • Nitrogen • Oxygen • Ammonia
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BioGas description
• Methane is the most important gas. – Combustible and therefore valuable as fuel. – Tasteless, odorless and colorless. – Lighter than air, safety bonus compared to
LPG. – Often people think they are smelling methane,
but are in fact smelling H2S. • Hydrogen Sulphide (H2S) is foul smelling
and corrosive gas.
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Organic Waste
• Organic def. – Relating to or derived from living matter.
• With the exception of a few substances all organic matter is suitable for BioGas production. Incl: – Fruit and Veg (and related waste). – Agri waste (Plant matter). – Manures. – Waste waters (containing organics).
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Triple Benefit of BioGas
• It is important to view the BioGas process with regard to it’s three benefits and not purely as fuel. 1. Combustible gas for fuel 2. Responsible waste disposal 3. Nutrient rich by-product used as fertilizer
• In rural applications also has major health benefit when compared to wood burning.
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Completing the cycle
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Anaerobic Digestion (AD)
• Organic matter broken down by micro-organisms in the absence of air.
• Air is toxic to bacteria. • Many different variable to control in order
to maintain efficiency. • Waste and bacteria housed in air-tight
tanks – digesters. • Importance of quality development.
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The potential of one cow
1 Dairy Cow
Electricity 791 kWh/Cow/Yr Value @ R1/kWh = R791
Heat 1144 kWh/Cow/Yr Value @ R0,25/kWh = R286
Solids 4 ton/Cow/yr Value of Phosphate R900
Liquid Effluent 14 ton/Cow/yr Value of N,K R1100
Potential value of ONE dairy cow = R3 077
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3 phases of AD
• Different bacteria play different roles. • Generally 3 phases happen concurrently.
1. Hydrolysis 2. Acidification 3. Methanogenesis
• Each stage requires specific bacteria. • Each bacteria requires correct conditions.
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3 phases of AD
Hydrolysis • Complex compounds made water soluble
Acidification • Formation of acids from simple
compounds (Acetic and other)
Methanogenesis • Acids converted into CH4 and CO2
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3 phases of AD
• Each of these phases requires their own set of parameters.
• What is good for one phase may not be good for the other.
• A fine balance is important to achieve best results.
• Phases can be done concurrently or separately.
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Variables
• A digester is considered a living organism. • NB parameters:
– Temperature – pH – Rate at which organic are loaded – Time spent in digester – Solid content – Degree of mixing – C:N ratio
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Variables
• Process parameters differ based on: – Feedstock – Digester design – Climate
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The stomach analogy
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The stomach analogy
• Easy to look at digester like a stomach. • We take in liquid and solids – output liquid,
solid and gas! • To keep our stomach’s healthy we need
the right amount of solid and liquid, acids and bases.
• Durban Curry = Just Flames, no gas.
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Digester Design
• As with most technologies, there are a variety of designs available to accomplish a task with varying degrees of success.
• Choice is based on: – School of thought – Budget – Required efficiency – Application
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Digester design
• Two main categories of digester: 1. Continuously fed 2. Batch fed
• Both of which can be: – Underground – Above ground
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Continuously Fed
• Waste added at consistent intervals. • Digester can hold 15-40 ‘intervals’. • When waste is added, slurry is removed. • Waste is generally in liquid form. • Solid content of 8-15%. • Commonly
– CSTR type – UASB type
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Batch Fed
• Waste in added, and then left to completely digest.
• For applications when waste not available every day.
• Digester is big enough for one ‘batch’ • Once digested, emptied and filled with
new waste. • Often higher solid content.
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Chinese fixed dome
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Indian Floating Drum
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Advanced Designs
• CSTR • UASB • Multi-stage reactors
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CSTR
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BioGas purification
• Reasons for purification: – H2S is poisonous in certain quantities. – H2S is a corrosive gas. – CO2 has no calorific value and therefore
consumes storage space for no benefit. – When H2S is combusted is releases harmful
SO2 into the atmosphere.
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Storage
• CO2 prohibits large volumes of storage. • If CO2 removed, biomethane can be
compressed and stored in the same way as natural gas. High energy req.
• Domestic applications may store up to 60% of daily produce.
• Commercial generally only 5%. • All depends on application.
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Uses of BioGas
• Fed into appliances for cooking fuel. • Scrubbed and used in CHP engine (elec.) • Scrubbed, Purified for natural gas pipeline. • Scrubbed, Purified, Compressed for
vehicle fuel (CNG) • Further compressed for easier and more
efficient use (LNG)
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Uses - Cooking
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Uses – Electricity Gen.
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Uses – Vehicle Fuel
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Why does BioGas suit SA?
• Can be very simple or very technical, depending on application.
• Cheap and effective means of rural energy • Brownie points and financial gain for
commercial installations. • Waste mitigation. • Ease dependency on National generator.
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Thank you!
Biogas is the answer, not a question!