On-farm Evaluation and Demonstration of Small-scale Biogas Technology
Anaerobic digestion of manure and other organic waste materials is an excellent potential approach to improve agricultural waste management by providing adequate pathogen reduction, as well as capturing high-quality energy and nutrient products. The application of biogas technology in the US is currently limited to large-scale, industrial agricultural operations due to the large capital costs and lack of adequately scaled technology for small-scale agricultural operations. Existing small-scale biogas technology from Asia provides a “platform” on which improved technology could be developed and deployed to the benefit of small farms in the United States. Many small, mixed horticulture and livestock farmers have indicated an interest in biogas applications that can serve as an alternative to composting, while supplying energy that can supplement or substitute for fossil fuel resources (LP gas, natural gas, etc.) they use on the farm or for on-farm processing.
The objectives of this project are to:
1. introduce farmers to the potential opportunities of small-scale biogas technology applications
2. develop an improved / modified design based on the various Asian technologies
3. deploy and evaluate pilot plants on three farms.
• Cooperating producers have been identified and three sites have been selected. A site visit to one farm was held in conjunction with a farm tour.
• An initial producer workshop was held and a project website with workshop materials and detailed background information was created: http://cff.wsu.edu/Project/smallscalebiogas/smallscalebiogas.html.
• A biogas project assessment tool was developed and provided to the farmers to determine sizing and feeding strategies for each site.
• The Project Director presented on small-scale biogas technology as part of a presentation on strategies for on-farm energy independence at the Harvesting Clean Energy VII Conference in Boise.
• An improved, small-scale biogas plant design was developed and an invention disclosure was submitted to the WSU Research Foundation.
• Testing of the improved design has been initiated.
BACKGROUND OF THE INVENTION
Biogas digester offers an all-in-one means to meet multi-environmental and economic requirements such as greenhouse gas reduction, renewable energy generation, organic wastes stabilization, odor elimination, pathogen removal and organic fertilizer production. In terms of application goal and scale, biogas digester can be categorized into two types, i.e. small-scale and medium/large-scale biogas digesters. The small-scale digester always has a limited working volume lower than 50 m3 and serves primarily for household heating and cooking energy generation; in comparison, medium/large-scale biogas digesters are designed for industrial and municipal organic wastes treatment with COD/BOD reduction as a primary purpose and biogas production as a tipping fee. In contrast to the intensive research and development of medium/large scale biogas digester for decades, the importance of small scale one has been largely ignored. While, along with the fossil energy stringency in recent years, the benefit of applying small-scale digester to meet household energy requirement in the course of household wastes stabilization and organic fertilizer production has stepped into the vision of modern farmer families.
As a matter of fact, small-scale digester has been broadly used as economical energy provider for household farmers in developing countries for decades. Several typical small-scale digesters designs broadly applied in China, India, Nepal and Vietnam, respectively. All of those small-scale digesters are actually following the same look comprising a domed column digester body linked in between influent and effluent ports. Accordingly, feedstock is pushed in from one end, biogas is collected from dome, and digested sludge as well as extra fermentation broth is removed from the other end of the digester. These analogies can be attributed to the common needs for household from different countries. Household users always desire their digesters to be small, simple and economical. The small digesters are all under the volume of 50 m3 and their construction costs are less than 150 US$ without external energy input but enough biogas output for adjacent household usage. However, the over simplicity in these digesters also incurs problems to household users. For cost-effectiveness concern, they are lack of heating and mixing mechanisms that are often compulsory in medium/large scale digesters, which result in slow biogas productivity and severe mass transfer limitations. Furthermore, congestion and crustation are also main factors causing a digester to be abandoned at a short lifetime. For this regard, our invention aims to provide a design capable of overcoming those conventional household digesters shortcomings while still keeping their benefits in terms of simplicity and economy. This disclosed design includes following advantages, i) a greenhouse temperature preserving strategy is taken to keep underground digester at high rate performance with no need of external energy input; ii) a biogas pressure driven fermentation broth auto-recirculation mechanics is exploited for the first time to realize mixing and inoculation in household digester with no extra system complexity; iii) a hydrolysis chamber is specially dedicated to separately hydrolyzing stalk materials from animal manure so as to avoid congestion and scum crustation problems; iv) a high surface inner digester wall is designed for high biomass retention in the form of biofilm; and v) a arc baffle is placed to prevent flow short-circuit. More importantly, detailed design guidelines are provided for potential users to fabricate such an advanced small digester.
BRIEF SUMMARY OF THE INVENTION
The subject invention provides a design and method for the efficient anaerobic digestion of household wastes in a small-scale anaerobic digester with simple configuration and no need of energy input. It is a further object of the present invention to provide a closed system apparatus and method to separately digest crop wastes before mixing its hydrolysate with the animal manure in fermentation chamber. It is still further object of the present invention to provide a closed system apparatus and method for methanation of livestock manure with solar energy in greenhouse in winter. It is still further object of the present invention to install an arc baffle to prevent short-circuit and dead zones in small-scale anaerobic digester. Further objects and advantages of the present invention will become apparent by reference to the following description of the preferred embodiment and appended drawings.
The present invention includes a column fermentation chamber, a domed gas chamber, influent and effluent chambers and a hydrolysis chamber. Animal manure and crop wastes are separately fed into the influent chamber and hydrolysis chamber, respectively. The biogas production and usage in the biogas chamber creates dynamic force driving automatic fermentation broth recirculation from effluent chamber into influent chamber. This recirculation also solves the poor inoculation problem in conventional small-digester design. This small-scale digester is proposed to be constructed beneath the frost depth under greenhouse floor.
In a specific embodiment, the digester includes an influent chamber that delivers collected animal manure through an influent tube to the bottom of fermentation chamber. A slope floor will facilitate the feedstock transport from influent to effluent ports along a specially designed arc baffle during the fermentation phase by gravity force or biogas pressure. The fermentation broth will be pressed by biogas pressure over the back valve from effluent chamber into hydrolysis chamber and then into the influent chamber to realize an auto-recirculation which brings hydrolysate produced in hydrolysis tank from straw materials and the rich inoculums accumulated in effluent chamber into fermentation chamber for enhanced biogas production. This auto-recirculation also breaks the layered mixed liquor structure in conventional household digester.
Impacts and Contributions/Outcomes
• More than 200 individuals (producers and others) in the Pacific Northwest have been introduced to the potential for small-scale applications of biogas technology as an alternative waste management / energy technology.
• Several producers have learned specific information about small-scale biogas technology and are prepared to fabricate and deploy a pilot biogas plant on their farms.
• An improved design has been developed that will provide far superior performance for applications of biogas technology in the US (and around the world).
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