Benefits
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Environmental Benefits
A modern anaerobic digester that is well run will produce significantly more energy than is consumed in the ongoing operation of the unit, including the transportation energy expended in collecting and transporting the feedstock that is used. This attractive energy balance compares favourably with some other forms of renewable energy. In addition, an anaerobic digester reduces greenhouse gas emissions of a farm operation through two primary mechanisms. Methane is a major green house gas - about 21 times more harmful than CO2 by volume - and is produced through the natural decomposition of manure. An anaerobic digester maximizes and captures methane that is produced, not allowing its release into the atmosphere. In addition, the biogas produced through anaerobic digestion is used as an energy source. This displaces fossil fuels that would otherwise have been used. The use of fossil fuels increases the net amount of atmospheric carbon dioxide present, contributing to the growing crisis of human-caused climate change. The use of biogas as a fuel results in no net increase in atmospheric carbon. Finally, animal manures have the potential to pollute land and ground water if disposed of incorrectly. The treatment of these materials through anaerobic digestion significantly reduces the pathogen load of manure, decreasing their environmental impact.
Economic and operational benefits
There are several operational benefits to a farm operation using anaerobic digestion. It represents an effective method for managing animal waste. It stabilizes manure, allowing for easier and longer storage, and reduces odour by approximately 80%. The process destroys virtually all weed seeds present, and creates a uniform end product that retains the nutrient properties of the material used. This creates a high quality fertilizer / soil amendment that can be applied with precision, either as is, or separated into liquid and solid fractions before use. The liquid fraction retains 2/3rds of the nutrients, and can be applied through traditional low cost liquid application infrastructure. The solid fraction has value as a soil amendment, or as a replacement for peat. The use of an anaerobic digester can be an effective step in meeting new regulatory requirements for the treatment of manure that are coming into force in jurisdictions across Canada.
There are four potential revenue streams associated with an on-farm anaerobic digester. Revenue can be generated from the sale of heat and the sale of electricity that is produced from the combustion of biogas. Alternately, this heat and electricity can be used on-farm, saving the cost of purchasing power or generating heat from fossil fuels. In addition to using manure as a feedstock, on-farm anaerobic digesters can benefit from adding suitable off-farm materials such as the waste from food and beverage processing industries into the feedstock mix. If regulations permit their use in an agricultural setting, an anaerobic digester operator can receive a fee to receive off-farm wastes, sometimes quite a substantial fee. There is also the future potential for generating revenue through the sale of carbon credits. However, Canada has not yet implemented a regulatory infrastructure to make domestic trading a reality, and access to foreign carbon credit markets by individuals might be a challenge.
As of yet, the financial feasibility of anaerobic digestion in Canada remains in question. The technology is gaining widespread acceptance and adoption in Europe due to generous financial incentives for green power. Until similar incentives are in place in Canada, or until the cost of power increases substantially (which may occur rapidly due to the growing imbalance between the demand for fossil fuels and the world's finite supply), anaerobic digestion is best contemplated where multiple outcomes are desired (ex: effective manure management to meet regulatory obligations, large quantity of heat required, quantifiable contribution to the environment, odour control) and where financial imperatives are not the only driver.
Synergies
Inputs and Outputs of the Technology:
The inputs to an anaerobic digester are the feedstock material (on-farm and off-farm wastes) and process heat and electricity. The outputs are methane-rich biogas, and a spent digestate slurry that has been stabilized and largely sanitized, which can be used as a nutrient-rich fertilizer / soil amendment. The biogas is combusted to produce heat and electricity. Approximately 20% the electricity produced is consumed to maintain the anaerobic digestion process itself, as well as a certain percent of the heat.
The primary synergy involved in anaerobic digestion involves its capacity to effectively treat manure and other organic wastes to meet increasingly strict environmental regulations, while simultaneously capturing methane for the production of renewable energy.
Potential and / or examples of hybrids:
It is interesting to note that the addition of glycerin to an anaerobic digester recipe has been shown to significantly increase the methane produced for a given volume of feedstock. Glycerin is the primary by-product in the production of biodiesel. However, there is no pressing reason for the co-location of a biodiesel facility with an anaerobic digester.
In addition, an on-farm anaerobic digester will generate a significant quantity of heat, which must be used in close proximity to the generating plant or is wasted. If heat is being produced with no available use, it may be feasible to use it in drying various biomass feedstocks in preparation for thermal processing. However, this cannot be considered a typical hybrid arrangement, with no known examples existing in North America.
A Simplified Financial Model
The following is a very rough financial assessment of an anaerobic digester project on a farm that operates with 500 dairy cows. It must be stressed that it can be extremely problematic to generalize about anaerobic digesters. There are a wide range of digester designs on the market, with widely varying operating parameters and costs. Other critical factors, such as the amount of useable manure collected per animal and the methane potential of this manure will also vary widely from farm to farm. The use of off-farm feedstocks in the mix will also change the operating parameters. With this in mind, the following financial model can only be taken as a gross approximation.
Assumptions:
- A dairy cow will produce 50 tonnes of usable manure feedstock per year.
- A tonne of diary manure feedstock will produce 25 m3 of biogas.
- There is 1.7 kilowatt-hours (kWh) of electricity generated per m3 of biogas combusted in a combined heat and power unit (assuming 30% conversion efficiency).
- There is 2 kWh of heat captured per m3 of biogas combusted in a combined heat and power unit.
- The electricity sold to the grid will receive 11.6 cents per kWh (weighted average), representative of the Ontario Standard Offer Program. This rate will typically be less in other provinces.
- The project will connect to the grid through net metering, and this amount is included in the overall capital costs. In some cases, there may be a substantial additional fee involved for the infrastructure needed to connect to the grid for the sale of electricity.
- Operating costs are assumed to be 1% of the capital costs of the digester.
- An anaerobic digester will require approximately 20% of the electricity it produces to sustain the anaerobic digester project.
Based on these assumptions, 500 dairy cows will generate 25 000 tonnes of manure feedstock per year, establishing the scale of the anaerobic digester. This manure will yield 625 000 m3 of biogas through anaerobic digestion. The combustion of this biogas will generate 1 062 500 kWh of electricity, and 1 250 000 kWh of heat when combusted in a combined heat and power unit. 212 500 kWh of electricity will be need to operate the anaerobic digester itself, leaving 850 000 kWh for sale to the grid.
Using the rough rules of thumb provided for capital costs that were provided in the previous section, a digester sized to process 25 000 tonnes of feedstock per year will cost between $1 250 000 and $1 750 000 (+/- 30%). Annual operating costs would be between $12 500 and $17 500.
Under this model, the anaerobic digester would generate approximately $98 600 annually from the sale of electricity. It is assumed that the spent digestate is needed for on-premises land application, so generates no revenue. Similarly, the heat generated by the system is assumed to not have an independent customer and so no revenue is attributable to the heat produced although it could displace the cost of generating heat from fossil fuels for on-farm use. This benefit is not captured in this simplified model, but could be important for some types of farms.
Taken all together, this anaerobic digester would generate between $81 100 and
$86 100 per year on a total capital cost of between $1 250 000 and $1 750 000 (+/- 30%), or roughly a 5% to 6.5% annual return, assuming that the digester is able to operate at 100% capacity without interruption throughout the year.
Not taken into account in this model is the time that will be required to operate the anaerobic digester, the start-up costs, and the costs of training. A moderately sized on-farm digester will require a minimum of an hour of dedicated attention per day.
Modified: 02-19-2008
