To evaluate biofuel systems it is important to be able to identify the type of equipment in use, and understand generally how that equipment works. The following section presents common equipment you are likely to encounter in a biofuel system along with important details.
In order to produce biogas from organic material it must be collected first. This is commonly accomplished through the use of one of the following systems:
- Flush - A flush system uses water to flush the waste material into a collection tank. This results in a heavily diluted solution but is much less labor intensive than the other systems making it the most economic.
- Scrape - A scrape system collects the waste by scraping it to a sump. This results in non diluted solutions in most operating conditions.
- Vacuum - A vacuum truck is used to collect the waste. This process can be slow and tedious but has the advantage of easy transportation to off-site locations.
- Loader - Front end loaders are used to collect and stack the waste along with other materials resulting in a dryer solution.
Anaerobic digesters use bacteria and heat in an oxygen free environment to convert volatile manure into usable methane gas. The digestion of manure occurs in four basic stages; hydrolysis, acidogenesis, acetogenesis, and methanogenesis. It is the last stage in which volatile methane gas is produced; therefore, it is important for the digester to have an appropriate manure retention time to allow the manure to fully decompose. The byproducts, nutrient rich digestate, can be used as an effective fertilizer. Anaerobic digesters can be separated into two different types, batch and continuous systems.
These digesters use a process in which organic materials are loaded and sealed into a reactor allowing them to digest. The retention time varies depending a wide range of factors but once the digestion is complete, the effluent is removed and reactors is loaded again, repeating the process. This provided an unsteady stream of bio-gas and effluent but is also much simpler to manage and construct making it ideal for small scale applications.
These digesters are continuously loaded with organic material and rely either the force of the new material being fed into the reactor or by a mechanical system to keep the material moving through. This produces a steady stream of bio-gas and effluent making it better suited for large scale applications. There are three types of continuous digesters:
- Vertical/Horizontal Tank Systems
- Plug Flow Systems
- Multiple Tank Systems
System for Using Biogas
The produced methane gas is most commonly used for power generation, heat generation, or flared off depending on the circumstance. The best systems uses a combination of power and heat generation that will reduce facility energy costs. These systems consist of a methane combustion engine which spins a generator to produce electricity. Excess heat from the engine is then collected using a heat transfer fluid and heat exchangers to provide heat for nearby end uses.
System for Using Effluent
The anaerobic microorganisms break down potential odor causing compounds. This offers a reduction in odors by up to 97 percent almost eliminating odors completely. Because the digestion process is anaerobic it kills almost all unwanted weeds and pathogens. The digestion process also reduces the volume of manure solids by up to 90 percent leaving a high quality concentrated fertilizer. This fertilizer can either be sold to local consumers or used on on-site depending on the size of the facility.