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Anaerobic digestion
Anaerobic digestion is a natural biological process involving the microbiological conversion of organic matter into methane in the absence of oxygen. It occurs throughout nature when high concentrations of wet organic matter are found in the absence of oxygen. It has been used in processing for over 150 years.
An efficient running digester will convert source feed stocks (manure, crop residue, energy crops, food processing waste, organic fraction of municipal solid waste, and industrial organics) into two streams, a nutrient-rich and stabilized slurry, and biogas that is roughly 65% methane. The biogas can be captured and combusted to create heat and electricity. A digester can be a significant and stable small-scale contributor of electricity to the grid.
Primary Benefit: biogas - primarily methane is produced that can be turned into electrical energy and sold
Other Benefits:
- Reduces O2 demand
- Reduced odors
- Fly control
- Reduced pathogens
- Reduction in weed seed germination in digestate
- Reduction in greenhouse gases
- Soil conditioner/fertilizer value
4 ingredients needed to produce biogas:
- Organic matter
- Bacteria
- Anaerobic (with oxygen) conditions
- Heat
The organic matter acts as the food source for the bacteria, with methane produced during digestion of the biomass. Potential sources of organic matter for farm-based biogas production include manure, crop wastes and residues, and dedicated energy crops.
Interesting statistics:
Each cubic meter of biogas produces approximately 23.2 MJ of energy, but this value can vary from 20-25 MJ/m3 depending on the methane content of the gas produced.
Biogas from digestion is typically 55-70% methane
4 Types of Biogas Generators
1.On-farm digesters using only manure and/or energy crops produced.
2.Centralized digesters using manure, energy crops and/or industrial organics from a wide range of sources.
3.Municipal sewage treatment digesters using municipal biosolids as a primary feedstock.
4.Waste water treatment systems used by food and beverage processing companies where biogas is produced as a by-product and either flared off or co-fired into boilers to produce steam.
More gas coming
Canadian Biogas production is currently at 8038 TJ, and expected to increase.Biogas Energy Supply Opportunities:
Storage systems have been employed to smooth out variations in gas production, gas quality and gas consumption. The storage component also acts as a reservoir, allowing downstream equipment to operate at a constant pressure.
The biogas may be applied in direct combustion systems (boilers, turbines, or fuel cells) for producing space heating, water heating, drying, absorption cooling, and steam production.
The gas used directly in gas turbines and fuel cells may produce electricity.
An alternative choice in biogas conversion is the use of stationary or mobile internal combustion engines which may result in shaft horsepower, cogeneration of electricity, and/or for transportation vehicles.
Another opportunity exists for sale of the biogas through injection into a natural gas pipeline. Biogas can be used readily in all applications designed for natural gas if cleaned of impurities.
Manure Feedstock
Canadian livestock currently produce an estimated 320 million tonnes of manure every year, with over 50% from beef cattle.
Not all manure is equal
Biogas yields differ between manure from different ***insert box, Biogas yield Potential of livestock manure, page 2, CFBMC CD - Biogas final draft updated dec'07.doc*** animals and can range from 19 to 96 m3 per tonne of manure.
Energy Crop Feedstock
Energy crops have been the primary feedstock for the rapid growth of digesters in Germany, where biogas now exceeds wind power in green energy production. What's the energy yield ***link to table, potential biogas yield of selected energy crops in Eastern ON, page 3, CFBMC CD - biogas final draft - updated Dec'07.doc*** potential in Eastern Canada?
Economics
Capital Costs
Capital costs for on-farm digesters are estimated at $50-75 per m3 of capacity +/-30%. These numbers will be more defined as we gain Canadian experience ***link to case study, Case study 1: Financial feasibility of 4 sizes of on-farm biogas digesters in Ontario, pages 5&6, CFBMC CD - biogas final draft updated dec'07.
Operating Costs
Management is key to success, but a good rule of thumb is that annual operation and maintenance costs are approximately 5% of the initial capital cost of the system.
Revenues
Revenues from anaerobic digestion come from the sale or use of the end product of biogas (for electricity and/or heat) and by-products like the solid sewage for use as fertilizer. Savings can also be had from reduced on-farm use of natural gas and propane, and bedding.
Farmers may also generate additional revenues by adding off-farm waste materials such as food processing wastes as co-feedstocks, as industry currently pays to dispose of these materials.
Highly Sustainable!
The energy output to input ratio for biofuels is the total amount of energy contained within the end product (the output), relative to the total amount of energy used in the production and processing of the crop (the input). The higher the ratio value, the higher the sustainability of the energy production system.
The energy output to input ratio of biogas is approximately 6:1, which is high compared to other bioenergies such as ethanol has a ratio of 1.25:1.
Biogas Production
The anaerobic digester is an airtight tank with heating coils and, in some systems, a mechanical mixer. As the feedstock is fed into the digester, it is heated to a temperature of 35°C or higher. At this temperature, bacteria produce biogas; which occurs in a two-step process.
The first step involves a group of anaerobic bacteria (acid formers) that produce organic acids from the initial organic degradation.
The second step involves a group of bacteria (methane formers) that break down the organic acids and produce methane in the process. As the gas is produced, it rises above the feedstock to the top of the digester where it is collected into the piping system.
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