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Anaerobic digestion is a biochemical process in which particular kinds of bacteria digest biomass in an oxygen-free environment. Several different types of bacteria work together to break down complex organic wastes in stages, resulting in the production of "biogas."
Feedstock to anaerobic digestion includes animal manure, crop residues, abattoir-wastes, industrial organic wastes, municipal solid waste, hazardous waste, sewage sludge and source-separated household organic waste. The largest sources of wastes are animal manure and crop residues that are derived from food production.
Anaerobic digestion involves the following stages; pre-treatment, primary storage, dilution, mixing and feeding, digestion, gas collection, solid-liquid separation, composting of digested solids and finally steam reforming of biogas for use in fuel cells to generate electricity. Unit operations involved are those for separation, size reduction (pulveriser) and solid-liquid separation.
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THE PROCESS
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Detailed description of each stage in Agni’s bio-methanisation process:
Stage 1: Hydrolysis
Hydrolysis Process – conversion of insoluble high molecular compounds (lignin, carbohydrates, fats) to lower molecular compounds
Stage 2: Acedogenesis/ Fermentation
Acidogenic (acid-forming/ fermentative) bacteria convert sugars, amino acids, and fatty acids to organic acids (e.g., propionic, formic, lactic, butyric, or succinic acids), alcohols and ketones (e.g., ethanol, methanol, glycerol& acetone), acetate, CO2 , and H2 .
Stage 3: Acetogenesis
In this stage the acetogenic bacteria (homoacetogenic bacteria) convert acids and alcohols into acetate, hydrogen and carbon dioxide.
Stage 4: Methanogenesis
Methane (CH4 ) is formed from acetate and hydrogen/carbon dioxide by methanogenic bacteria. Methanogens grow best in a pH range of 7.5-8.5 and are very sensitive to O2 |
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Biogas produced in anaerobic digesters consists of methane (50%-80%), carbon dioxide (20%-50%), and trace levels of other gases such as hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulphide.
Methane can be used whole or in part to generate electricity for use, with the excess sold to the electric grid.
The financial costs of anaerobic digestion are quite high but the source of income coming from the sale of electricity, heat and digestate allow important benefits.
Agni’s waste to energy systems can provide: -
- High process and high net energy efficiencies, therefore better economics
- More stable digestion, allowing higher throughput
- Higher methane content in the biogas (±70% methane vs. ±60% for conventional technologies), resulting in better fuel value for on-site use
- Higher digestion efficiency, thus more methane recovered
- Natural pathogen destruction, therefore safer use of digested solids for land fertilization and reclamation
Materials that can be recovered for recycling from MSW to generate extra revenue
- Aluminium
- Paper
- Plastics
- Glass
- Ferrous metals (Iron & Steel)
- Non-ferrous Metals
- Yard Wastes
- Construction and demolition wastes
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