Term | Meaning |
Alcohol |
A volatile liquid fuel that is produced throught the fermentation of sugars and starches. |
Biodiesel | A form of biofuel usually produced from oil seed crops that is used in diesel engines. |
Biofuel | The fuel that is produced from biomass. |
Biomass | Biological material that can be converted into usable fuel. |
Cellulose | Fiber contained in organic material such as plant or trees. |
Cetane Rating | A measure of diesel's combustion quality. |
Digestate | The solid remnants of the original input material to the digesters that the microbes cannot use |
Effluent | The outflowing material drawn from the digester that is rich in nutrients and is offten reffered to as sludge. |
Energy Balance | The net difference between the energy needed to produce a fuel and the energy the produced fuel contains. |
Ethanol | Alcohol consisting of two carbon atoms per molecule, typically made from fermented orn starth or sugar cane. Also known as 'grain alcohol'. |
Ethers | A fuel consisiting of alcohol and isobutylene. |
Lignin | A material contained in biomass which is very energy dense. |
Methanol | Alcohol consisting of one carbon atom per molecule, typically made from natural gas. Also known as 'wood alcohol'. |
Ocatane | A measure of a fuels resistance to self detonation. |
Volatility | A measure of fuels tendancy to evaporate. |
Collection/Pre-Mixing System |
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:
|
Digester System |
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. Batch DigestersThese 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. Continuous DigestersThese 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:
|
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. |
Temperature |
There are two common temperatures that anaerobic digesters will operate at depending on the species of methanogens used.
|
Solids Content |
The solids content of the organic material can represents a large role in determining how the rest of the system is designed. The three most common operation parameters associated with solids contents is as follows:
|
Number of Stages | The number of stages a digester has can have a significant impact on the performance of the system. Digester can be either single stage or multistage. More stages allows for more fine tuning of the microorganisms and will yield a greater conversion efficiency. |
Walkthrough Checklist
U.S. DOE Energy Efficiency and Renewable Energy
The Midwest Rural Energy Council
ATTRA - National Sustainable Agriculture Information Service
UCS - Union of Concerned Scientists
NREL - National Renewable Energy Laboratory