Proposal for the development of a Biogas plant
1. Our Needs…………………………………………………………… 3
2. The Biogas process…………………………………………………...4
4. Resources and Acknowledgements…………………………………..10
1. Our needs
The process of transforming waste to energy is not a new idea. Research has shown that systems are already implemented in the USA, China, India and many other countries.
The implementation of small scale biogas (mixture of CH4 also known as methane and CO2) plants for farm or rural communities are however something that is fairly new to South Africa. One of the documented implementations of a bio-gas plant is at Maphephetheni in Kwazulu Natal
Not only do biogas digesters meet the thermal energy needs of communities, but they also have significant other benefits, such as:
- Improved air quality (the amount of smoke released into the atmosphere is reduced)
- Improved health and reduce respiratory elements
- Better management of animal dung and human excrement
- Reduce ground water pollution
- Reduce deforestation and resulting soil erosion
- Reduce depletion of solar nutrients
- Reduce green house gas emissions
- The slurry provides an excellent fertiliser and thereby increases crop production, or can be sold to generate income.
- Electricity can be produced that can be used to power communities or that can be used to generate income. The income can be used to fund other projects to improve the living standards of under developed communities.
As a team, we need to consider various aspects before we choose the optimum design or unit that suits our needs. The following is a list of factors that we must consider when choosing a methane plant implementation.
- The system must be compatible with the environment – Proper knowledge of the site, and environment related data. (This is crucial – choosing incorrect or non-optimum equipment is one of the main reasons a project like this will fail).
- Cost of the plant equipment and development
- Accessibility of the site – (Especially for equipment during the building phase)
- Duration of the development- and building phase
- Plant power system (Electricity).
- The plant must adhere to certain industry standards (esp. regarding the safety of the plant)
- Cost of maintaining the plant
- Control of the plant by the community (Training programs)
- Environmental issues or problems and how this is to be managed.
|bio-gas process diagram|
- Manure and waste collection
- Anaerobic Digester
- Effluent storage
- Gas handling
- Gas utilization
A brief discussion of the different stages follows.
2-1.1 Manure Collection
Livestock facilities use manure management systems to collect and store manure because of sanitary, environmental, and farm operational considerations. Manure is collected and stored as liquids, slurries, semi-solids, or solids. Note that the manure collection can also be implemented at a sewerage plant, were a flow or scrape system can be implemented to collect the waste.
The following discussion is intended for farm use, but it provides a guideline as to the conditions the waste can or must be in, when implementing a biogas plant.
Manure is excreted with a solids content of 8 to 25 percent, depending upon animal type. It can be diluted by various process waters or thickened by air drying or by adding bedding materials.
— Liquid Manure.
Manure handled as a liquid has been diluted to a solids content of less than 5 percent. This manure is typically “flushed” from where it is excreted, using fresh or recycled water. The manure and flush water can be pumped to treatment and storage tanks, ponds, lagoons, or other suitable structures before land application. Liquid manure systems may be adapted for biogas production and energy recovery in “warm” climates. In colder climates, biogas recovery can be used, but is usually limited to gas flaring for odour control.
— Slurry Manure.
Manure handled as slurry has been diluted to a solids content of about 5 to 10 percent. Slurry manure is usually collected by a mechanical “scraper” system. This manure can be pumped, and is often treated or stored in tanks, ponds, or lagoons prior to land application. Some amount of water is generally mixed with the manure to create slurry. For example, spilled drinking water mixes with pig manure to create slurry. Manure managed in this manner may be used for biogas recovery and energy production, depending on climate and dilution factors.
— Semi-Solid Manure.
Manure handled as a semi-solid has a solids content of 10 to 20 percent. This manure is typically scraped. Water is not added to the manure, and the manure is typically stored until it is spread on local fields. Fresh scraped manure (less than one week old) can be used for biogas and energy production in all climates, because it can be heated to promote bacterial growth.
— Solid Manure.
Manure with a solids content of greater than 20 percent is handled as a solid by a scoop loader. Aged solid manure or manure that is left “unmanaged” (i.e., is left in the pasture where it is deposited by the animals) or allowed to dry is not suitable for biogas recovery.
2-1.2 Digester Types
The digester is the component of the manure management system that optimizes naturally occurring anaerobic bacteria to decompose and treat the manure while producing biogas. Digesters are covered with an air-tight impermeable cover to trap the biogas for on-farm energy use. The choice of which digester to use is driven by the existing (or planned) manure handling system at the facility. This is something that must be addressed prior to deciding on the plant layout. The digester must be designed to operate as part of the facility’s operations.
— Covered Lagoon Digester:
Covered lagoons are used to treat and produce biogas from liquid manure with less than 3 percent solids. Generally, large lagoon volumes are required, preferably with depths greater than 12 feet (about 3.66 m). The typical volume of the required lagoon can be roughly estimated by multiplying the daily manure flush volume by 40 to 60. This is however a rough estimate and it is therefore necessary that the chemical engineering team produce a more conclusive and precise analysis. Covered lagoons for energy recovery are compatible with flush manure systems (see page 5 on liquid manure systems) in warm climates. Covered lagoons may be used in cold climates for seasonal biogas recovery and odour control (gas flaring). There are two types of covers, bank-to-bank and modular. A bank-to-bank cover is used in moderate to heavy rainfall regions. A modular cover is used for arid regions. Figure 2.1.1 illustrates a modular floating cover for lagoon applications. Typically, multiple modules cover the lagoon surface and can be fabricated from various materials.
— Complete Mix Digester:
Complete mix digesters are engineered tanks, above or below ground that treats slurry manure with a solids concentration in the range of 3 to 10 percent. These structures require less land than lagoons and are heated. Complete mix digesters are compatible with combinations of scraped and flushed manure. This is attractive in the sense that it allows for a variety of waste sources to be implemented in the project.
— Plug Flow Digester:
Plug flow digesters are engineered, heated, rectangular tanks that treat scraped dairy manure with a range of 11 to 13 percent total solids. Swine manure cannot be treated with a plug flow digester due to its lack of fibre.
— Fixed Film Digester.
Fixed-film digesters consist of a tank filled with plastic media. The media supports a thin layer of anaerobic bacteria called “bio film” (hence the term "fixed-film"). As the waste manure passes through the media, biogas is produced. Like covered lagoon digesters fixed-film digesters are best suited for dilute waste streams typically associated with flush manure handling or pit recharge manure collection. Fixed-film digesters can be used for both dairy and swine wastes. However, separation of dairy manure is required to remove slowly degradable solids.
2-1.3 Effluent Storage
The products of the anaerobic digestion of manure in digesters are biogas and effluent. The effluent is a stabilized organic solution that has value as a fertilizer and other potential uses. Waste storage facilities are required to store treated effluent because the nutrients in the effluent cannot be applied to land and crops year round.
This can be another form of income result of the project. A detailed agricultural analysis has to be performed to see how feasible it is to consider implementing this part of the project.
The size of the storage facility and storage period must be adequate to meet farm and community requirements during the non-growing season. Facilities with longer storage periods allow flexibility in managing the waste to accommodate weather changes, equipment availability and breakdown, and overall operation management.
2-1.4 Gas Handling
A gas handling system removes biogas from the digester and transports it to the end-use, such as an engine or flange. Gas handling includes: piping; gas pump or blower; gas meter; pressure regulator; and condensate drain(s). It is to be noted that this is the specifics for a small scale farm-implementation of a biogas system.
Biogas produced in the digester is trapped under an airtight cover placed over the digester. The biogas is removed by pulling a slight vacuum on the collection pipe (e.g., by connecting a gas pump/blower to the end of the pipe), which draws the collected gas from under the cover. A gas meter is used to monitor the gas flow rate. Sometimes a gas scrubber is needed to clean or “scrub” the biogas of corrosive compounds contained in the biogas (e.g., hydrogen sulphide). Warm biogas cools as it travels through the piping and water vapour in the gas condenses. A condensate drain(s) removes the condensate produced.
A detailed plant safety analysis has to be performed, because of the fact that the gas is highly flammable, that could result in a catastrophe if no proper planning is done.
2-1.5 Gas Utilization
Recovered biogas can be utilized in a variety of ways. The recovered gas is 60 - 80 percent methane, with a heating value of approximately 600 -800 Btu/ft3. Gas of this quality can be used to generate electricity; it may be used as fuel for a boiler, space heater, or refrigeration equipment; or it may be directly combusted as a cooking and lighting fuel. Electricity can be generated for on-farm use or for sale to the local electric power grid. The most common technology for generating electricity is an internal combustion engine with a generator. The predicted gas flow rate and the operating plan are used to size the electricity generation equipment. It is to be noted that generators are pricy equipment, and it is electricity is to be harvested from the project.
Engine-generator sets are available in many sizes. Some brands have a long history of reliable operation when fuelled by biogas. Electricity generated in this manner can replace energy purchased from the local utility, or can be sold directly to the local electricity supply system. In addition, waste heat from these engines can provide heating or hot water for farm or community use.
Biogas can also be used directly on-site as a fuel for facility operations. Equipment that normally uses propane or natural gas can be modified to use biogas. Such equipment includes boilers, heaters, and chillers.
This provides a way in which the facility can be partially self sustainable.
Before we choose to implement one of the above systems, we must first do a survey that covers the following aspects.
- What type of waste are we working with.(solid contents especially)
- How will the waste be transported to the plant? (This is directly related to plant location)
- How the products of the plant is to be utilized.
The following figure illustrates just how the waste and waste management practices affect the feasibility and choice of a biogas system.
A proposal for the system in a region like Stellenbosch is ideal. There is a local sewerage plant, and the nearby farming community can provide agricultural waste. If sewerage is to be made the main “waste source”, then the total solid estimate is about 0 to 10 % in the waste. This automatically allows us to consider “pump” handling, and a covered lagoon or complete mix- type digester.
My opinion would be to have the biogas plant as close as possible to the sewerage plant, for then extra costs regarding pipelines for sewerage transfer and odour control problems is cut out.
4. Resources and Acknowledgements
- Agricultural Anaerobic Digestion, John D. Ewing
- AgSTAR handbook: A manual for developing Biogas systems at commercial farms in the USA
- AgSTART paper: Market opportunities for Biogas recovery systems