Gas-holder is usually of flexible material, therefore to be protected against weather. Anaerobic digesters are most adaptable to farms that collect large amounts of manure as a liquid or slurry. Commercial electricity generation systems that use biogas consist of an internal combustion engine, a generator, a control system, and an optional heat recovery system. Biogas can then be used to generate electricity, as a boiler fuel for space or water heating, upgraded to natural gas pipeline quality, or for a variety of other uses. Continuous biogas designs are filled and emptied regularly — normally daily. Other factors include expenses for engine oil changes and minor repairs, and maintenance such as engine overhauls and sludge removal.
Suspended Media Digesters: In these digesters, microbes are suspended in a constant upward flow of liquid. The design of these systems is critical, and they need to be carefully designed by experts to ensure that the process will actually digest the waste material under various operating conditions at the site. The slowly digested straw-type material is fed in about twice a year as a batch load. A — Floating drum plant B — Fixed dome plant C -Fixed-dome with separate gasholder. The unit can be operated as a continuous flow plant with no compensating tank. Ferro-cement Plants The ferro-cement type of construction can be applied either as a self-supporting shell or an earth-pit lining.
Liquid retention times can be as short as five days. D — Balloon type of biogas design E — Channel typedigester with folia and sunshade. They hold manure in the absence of oxygen and maintain the proper conditions for methane forming microorganisms methanogens to grow. Microorganisms form biofilms around the larger particles, and methane formers stay in the digester. Since sludge is stored in lagoons for up to 20 years, also remain in the covered lagoon for up to 20 years.
Biogas production is maintained by adjusting volume so that liquids remain in the digester for 20 days to 30 days. One drawback to fixed film digesters is that manure solids can plug the media. The dung is added and removed regularly. The continuous types of biogas designs are more suitable for rural households as the necessary ongoing work fits better into the daily round. Continuous plants Continuous plants are fed and emptied continuously.
This is not always a bad thing if you consider the added material may also be biodegradable. This material prevents oxygen exposure, thus allowing chemical reactions and microbes to act upon the waste, and encouraging an uncontrolled process of biomass decay. Recommended retention time is 15 days to 20 days. The gas pressure is kept constant by the floating gasholder. It has established some production bases in several provinces or cities such as Shanghai, Zhejiang, Jiangsu, Anhui, etc. Since they are not heated, the temperature of covered lagoons follows seasonal patterns.
Intermittent mixing means the tank is stirred during feeding and only occasionally between feedings. The cycle is repeated up to four times a day for nearly constant gas production. Continuous plants empty automatically through the overflow. A fixed film digester Figure 8 is essentially a column packed with media, such as wood chips or small plastic rings. Methanogens flow out of the digester with the displaced liquid.
Passive Systems Covered lagoon Figure 2. Plug Flow Digester Figure 6. Assessing Resource Availability For either an anaerobic digestion or landfill gas recovery system, the following resources need to be assessed. The nutrient-rich solids resulting from the digestion can be used as a fertiliser subsequently. Landfill gas is about 40% to 60% methane, with the remainder consisting mostly of carbon dioxide. Mixing pit varies in size and shape according to the nature of substrate.
Digester size can be an issue at lower solids concentrations. Incoming liquid displaces volume in the digester, and an equal amount of liquid flows out. In general, the system needs to be large, such as a contained animal feeding operation or a zoo. A biogas design option emerging in the last 10 years has been that of the community biogas plant, providing both a convenient waste disposal route for organic wastes ad a source of income to farmers and residents alike within the community. Mixed Plug Flow Digester: This is a patented variation on a plug flow digester in which manure flows down a hairpin raceway Figure 6. Clearly the assumptions that I gleaned were wrong.