Technology
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State of the Technology
Anaerobic digestion can be considered a mature technology, having undergone several decades of commercial development, primarily in Europe. All anaerobic digesters are fundamentally a sealed, airless chamber (or series of chambers) that creates an optimum environment for the anaerobic bacteria to break down the organic matter that is present. Despite the maturity of the technology, there is as yet little convergence in anaerobic digestion technology, with a wide variety of designs and philosophies on the market, exhibiting widely varying degrees of capacities, prices, and operating parameters. These many competing designs fall under two broad categories:
Mesophilic digesters operate at 30-35°C optimal range, with the feedstock remaining in the digester for approximately 15-30 days. These digesters are more robust, and require less process heat to maintain, but typically involve larger chambers, have less biogas output, and do not sanitize the feedstock as thoroughly.
Thermophilic digesters operate at 50-55°C optimal range, with the feedstock remaining in the digester for 10-15 days. Advantages include slightly higher biogas production, faster throughput, and more complete sanitization, but are more expensive, require more process heat, and are less stable.
In addition, digester designs can be classified according to their type of throughput (batch, continues flow, or plug flow), and number of stages they utilize (one, two, or more). Other design considerations include the solids content it is designed to handle (liquids, slurries), capacity, orientation (vertical or horizontal) and mixing mechanism utilized.
Given the wide variety of competing design philosophies, and the large number of site-specific variables that need to be taken into account (type of livestock, numbers of animals, livestock housing arrangements, existing manure handling methods, etc.), discussions with industry experts (consultants/engineers, technology providers, government experts) will be needed to begin pinpointing particular design options to meet your needs.
Suitable Feedstocks
There are a wide range of potential feedstocks for anaerobic digestion, from both agricultural and post-consumer sources.
The primary agricultural feedstock is manure from cattle, swine and poultry. Animal manure, as excreted, is an excellent feedstock for the production of biogas. However, the manure is not available as excreted, but will be subject to change through collection and storage practices, and thus what will actually be available can more properly be called manure feedstock, which will have integrated additional materials such as bedding material, waste feed, soil, and potentially significant amounts of water, and will also be subject to degradation while stored. Manure feedstock will therefore not have the methane potential of pure manure.
Crop materials (residues and purpose grown crops) are not considered a primary feedstock for anaerobic digestion, but can be valuable additions in a recipe, with the ability to balance importance parameters of the overall feedstock mix.
In addition to agricultural materials, it can be valuable to include off-farm feedstocks in an anaerobic digester, where regulations permit. These feedstocks, in addition to their methane potential, could have an additional benefit in that the operator of the anaerobic digester could be paid a fee ("tipping fees") to receive and process these feedstocks. The primary off-farm feedstock to be considered is industrial organics - organic waste from the food and beverage processing industry. These feedstocks can have excellent methane potential, and often have significant tipping fees attached. Other potential feedstocks include the organic fraction of municipal waste streams (were available), yard and garden waste, and septage and biosolids (human wastes). It must be noted that in many Canadian jurisdictions, regulations currently do not permit the acceptance of off-farm wastes into an agricultural setting.
Anaerobic digestion is a complex/sensitive biological process requiring a feeding process properly setup/controlled. A digester represents a fragile environment optimized for a specific group of anaerobic bacteria. A carefully controlled equilibrium is needed to maintain the health of these bacteria. Deviation from optimum can rapidly lead to a negative feedback loop that cause total reactor failure, which will necessitate a lengthy period to re-start the process.
Anaerobic digesters must be fed a consistent feedstock mix, taking into account a number of parameters including pH level, carbon to nitrogen ratio, % volatile solids, % total solids, and particle size. Changes to the feedstock recipe can only be made carefully and slowly. The feed rate must also be closely managed. In short, the feeding of an anaerobic digester must be approached with the same diligence as the feeding of livestock.
Challenges
The primary challenge of on-farm anaerobic digesters in Canada is their unproven economics. Even in Ontario, where a premium rate of 11 cents per kWh is available for electricity delivered from an anaerobic digester project, such a venture is generally considered undesirable from a return on investment perspective. In Europe, the widespread adoption of anaerobic digestion has been driven by considerable financial incentives not in place in Canada.
The other major challenge has been the historical failure rate of on-farm anaerobic digester projects in North America, which some have estimated to be in the 80% range. An anaerobic digester is a complex entity, requiring considerable know-how, absolute consistency, and ongoing demands of time and attention to operate successfully. If they are allowed to deviate from optimum, they can quickly deteriorate to total failure, requiring a lengthy restart. Considering the questionable economics of anaerobic digestion in Canada, maximum output of biogas is required for virtually the entire year to achieve sufficient returns to justify the cost and effort. This has proved difficult to achieve. Modern systems can incorporate a high degree of automation, lessening the operating burden, but such automation adds to the capital costs involved.
Other challenges include the significant capital and operating costs involved, the lack of an established base of anaerobic digesters in Canada, the potential for environmentally harmful emissions resulting from the combustion of biogas (such as sulphur related compounds), the increase in traffic a project can entail if it includes off-farm feedstocks, and the noise an anaerobic digester facility can introduce into a rural environment.
Costs
In North America, with a large number of consultants and technology providers offering anaerobic digester solutions, but as yet a very small base of successful installations, each project must be approached as a custom design. Canadian technology providers have proven reluctant to offer generalizations about pricing for systems, given the great number of variables that must be taken into account when designing an anaerobic digester. With this in mind, here are some rough approximations for costs:
WARNING: All figures provided below can only be considered very rough rules of thumb, and should not be used beyond pre-feasibility considerations. | ||
| Rough Costs Estimates | Examples |
Capital Costs | It can be estimated as roughly $50 - $70 per tonne of feedstock that can be processed on an annual basis, plus/minus 30%. | For comparison purposes, a dairy cow will produce roughly 50 tonnes of manure feedstock per year, a hog will produce 1.3 tonnes, a laying hen will produce 0.04 tonnes, and a broiler will produce 0.03 tonnes. |
Electrical Generation Equipment | It represents roughly 25% of this overall figure. |
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Annual Operating Costs | Costs were seen to be between $500 and $1500 per m3 of reactor volume. |
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Annual Maintenance Costs | Costs are roughly 3% of the budget for processing equipment and 0.5% of the budget for other capital items. |
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Source: Goodfellow Agricola and Thorington Corporation
Again, it must be stressed that all figures provided above can only be considered very rough rules of thumb, and should not be used beyond pre-feasibility considerations.
Canadian Technology Providers
Genysis BioGas, Ottawa
www.kellerengineering.com, Benjamin Strehler, 613-224-1594
Rentec Renewable Energy technologies Inc., Peterborough
www.rentec.ca, 519-913-0065, 416-850-4427
BioTerre Systems Inc., Sherbrooke
www.bioterre.com, Richard Royer, 819-567-3871 (ext. 2256)
Clear Green, Saskatoon
www.clear-green.com, Rick Valette, 306-931-3810
Highmark Renewables Inc., Vegreville Alberta
www.highmark.ca, Mike Kotelko 780-768-2466, Xiao-Mei Li 780-450-5290
ECB Enviro North America, Fort MacLeod
www.ecbna.com, Thane Hurlbert, 403-553-4255
- ECB Enviro also offers a pre-treatment technology (thermo-hydrolysis) that enables the processing of specified risk materials so that they can be used for feedstock, www.biorefinex.com
Modified: 02-19-2008
