Page 291 - Synthetic Fuels Handbook

FUELS FROM CROPS 277

In recent years, increasing awareness that anaerobic digesters can help control the dis-
posal and odor of animal waste has stimulated renewed interest in the technology. New
digesters now are being built because they effectively eliminate the environmental hazards
of dairy farms and other animal feedlots.
Anaerobic digester systems can reduce fecal coliform bacteria in manure by more than
99 percent, virtually eliminating a major source of water pollution. Separation of the solids
during the digester process removes about 25 percent of the nutrients from manure, and
the solids can be sold out of the drainage basin where nutrient loading may be a problem.
In addition, the digester’s ability to produce and capture methane from the manure reduces
the amount of methane that otherwise would enter the atmosphere. Scientists have targeted
methane gas in the atmosphere as a contributor to global climate change.
Controlled anaerobic digestion requires an airtight chamber, called a digester. To pro-
mote bacterial activity, the digester must maintain a temperature of at least 68°F. Using
higher temperatures, up to 150°F, shortens processing time and reduces the required vol-
ume of the tank by 25 to 40 percent. However, there are more species of anaerobic bacteria
that thrive in the temperature range of a standard design (mesophillic bacteria) than there
are species that thrive at higher temperatures (thermophillic bacteria). High-temperature
digesters also are more prone to upset because of temperature fluctuations and their suc-
cessful operation requires close monitoring and diligent maintenance.
The biogas produced in a digester (digester gas) is actually a mixture of gases, with meth-
ane and carbon dioxide making up more than 90 percent of the total. Biogas typically contains
smaller amounts of hydrogen sulfide, nitrogen, hydrogen, methyl mercaptans, and oxygen.
Methane is a combustible gas. The energy content of digester gas depends on the amount
of methane it contains. Methane content varies from about 55 to 80 percent. Typical digester
3
gas, with a methane concentration of 65 percent, contains about 600 Btu/ft of energy.
There are three basic digester designs and all of them can trap methane and reduce
fecal coliform bacteria, but they differ in cost, climate suitability, and the concentration of
manure solids they can digest.
A covered lagoon digester, as the name suggests, consists of a manure storage lagoon
with a cover. The cover traps gas produced during decomposition of the manure. This type
of digester is the least expensive of the three.
Covering a manure storage lagoon is a simple form of digester technology suitable for
liquid manure with less than 3 percent solids. For this type of digester, an impermeable
floating cover of industrial fabric covers all or part of the lagoon. A concrete footing along
the edge of the lagoon holds the cover in place with an airtight seal. Methane produced in
the lagoon collects under the cover. A suction pipe extracts the gas for use. Covered lagoon
digesters require large lagoon volumes and a warm climate. Covered lagoons have low
capital cost, but these systems are not suitable for locations in cooler climates or locations
where a high water table exists.
A complete mix digester converts organic waste to biogas in a heated tank above or below
ground. A mechanical or gas mixer keeps the solids in suspension. Complete mix digesters
are expensive to construct and cost more than plug-flow digesters to operate and maintain.
Complete mix digesters are suitable for larger manure volumes having solids concentra-
tion of 3 to 10 percent. The reactor is a circular steel or poured concrete container. During
the digestion process, the manure slurry is continuously mixed to keep the solids in sus-
pension. Biogas accumulates at the top of the digester. The biogas can be used as fuel for
an engine-generator to produce electricity or as boiler fuel to produce steam. Using waste
heat from the engine or boiler to warm the slurry in the digester reduces retention time to
less than 20 days.
Plug-flow digesters are suitable for ruminant animal manure that has a solids concen-
tration of 11 to 13 percent. A typical design for a plug-flow system includes a manure
collection system, a mixing pit, and the digester itself. In the mixing pit, the addition of