Technology Primer

Definitons
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.
Equipment

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:

  • 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.

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 Digesters

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.

Continuous Digesters

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.

Performance Relationships
Temperature

There are two common temperatures that anaerobic digesters will operate at depending on the species of methanogens used.

  • Mesophilic - In the mesophilic range, mesophiles are the primary microorganisms and digestion reaches it optimum at around 98oF.
  • Thermophilic - In the thermophilic range, thermophiles are the primary microorganism and digestion reaches its optimum at around 130oF.  The rate of decomposition and bio-gas production in this range are much greater than in the mesophillic range.
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:

  • High-Solids (Dry)
  • High-Solids (Wet)
  • Low - Solids (Wet)
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. 

Common Oppertunities

Additional Information

Walkthrough Checklist

  •  Is there a suitable waste stream resource to justify a biofuel production system?
  •  Is there a non economic reason the facility would want a biofuel production system?

Additional Resources

U.S. DOE Energy Efficiency and Renewable Energy

 

The Minnesota Project

 

The Midwest Rural Energy Council

 

ATTRA - National Sustainable Agriculture Information Service

 

UCS - Union of Concerned Scientists

 

NREL - National Renewable Energy Laboratory