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is an Anaerobic Digester?
An Anaerobic Digester is a device for optimizing the anaerobic digestion of biomass and/or animal manure, and possibly to recover biogas
also referred to as BioMethane for energy production. Digester types include batch, complete mix, continuous flow (horizontal or plug-flow, multiple-tank, and vertical tank), and covered lagoon.
What is Anaerobic Digestion?
Anaerobic digestion is a biological process that produces a gas principally composed of methane (CH4) and carbon dioxide (CO2) otherwise known as biogas. These gases are produced from organic wastes such as livestock manure, food processing waste, etc.
Anaerobic processes could either occur naturally or in a controlled environment such as a biogas plant. Organic waste such as livestock manure and various types of bacteria are put in an airtight container called digester so the process could occur. Depending on the waste feedstock and the system design, biogas is typically 55 to 75 percent pure methane. State-of-the-art systems report producing biogas that is more than 95 percent pure methane.
EPA AgSTAR Program Background
U.S. EPA AgSTAR is an outreach program designed to reduce methane emissions
from livestock waste management operations by promoting the use of biogas
recovery systems. A biogas recovery system is an anaerobic digester with
biogas capture and combustion to produce electricity, heat or hot water.
Biogas recovery systems are effective at confined livestock facilities that
handle manure as liquids and slurries, typically swine and dairy farms.
Anaerobic digester technologies provide enhanced environmental and financial
performance when compared to traditional waste management systems such as
manure storages and lagoons. Anaerobic digesters are particularly effective
in reducing methane emissions but also provide other air and water pollution
control opportunities. AgSTAR provides an array of information and tools
designed to assist producers in the evaluation and implementation these
digester extension events and conferences
project development tools and industry listings
characterizations for digesters and conventional waste management
Operating a toll free
recognition for voluntary environmental initiatives
federal and state renewable energy, agricultural, and environmental
Methane Emissions from Animal Waste
emissions occur whenever animal waste is managed in anaerobic conditions.
Liquid manure management systems, such as ponds, anaerobic lagoons, and
holding tanks create oxygen free environments that promote methane
production. Manure deposited on fields and pastures, or otherwise handled in
a dry form, produces insignificant amounts of methane. Currently, livestock
waste contributes about 8 percent of human-related methane emissions in the
Given the trend toward larger farms, liquid manure management is expected to
increase. For more information on international emissions, projections, and
mitigation costs, see International
Methane is a gas that contains molecules of methane with one atom of
carbon and four atoms of hydrogen (CH4 ). It is the major component of the
"natural" gas used in many homes for cooking and heating. It is
odorless, colorless, and yields about 1,000 British Thermal Units (Btu)
[252 kilocalories (kcal)] of heat energy per cubic foot (0.028 cubic
meters) when burned. Natural gas is a fossil fuel that was created eons
ago by the anaerobic decomposition of organic materials. It is often found
in association with oil and coal.
The same types of anaerobic bacteria that produced natural gas also
produce methane today. Anaerobic bacteria are some of the oldest forms of
life on earth. They evolved before the photosynthesis of green plants
released large quantities of oxygen into the atmosphere. Anaerobic
bacteria break down or "digest" organic material in the absence
of oxygen and produce "biogas" as a waste product. (Aerobic
decomposition, or composting, requires large amounts of oxygen and
produces heat.) Anaerobic decomposition occurs naturally in swamps,
water-logged soils and rice fields, deep bodies of water, and in the
digestive systems of termites and large animals. Anaerobic processes can
be managed in a "digester" (an airtight tank) or a covered
lagoon (a pond used to store manure) for waste treatment. The primary
benefits of anaerobic digestion are nutrient recycling, waste treatment,
and odor control. Except in very large systems, biogas production is a
highly useful but secondary benefit.
Biogas produced in anaerobic digesters consists of methane (50%-80%),
carbon dioxide (20%-50%), and trace levels of other gases such as
hydrogen, carbon monoxide, nitrogen, oxygen, and hydrogen sulfide. The
relative percentage of these gases in biogas depends on the feed material
and management of the process. When burned, a cubic foot (0.028 cubic
meters) of biogas yields about 10 Btu (2.52 kcal) of heat energy per
percentage of methane composition. For example, biogas composed of 65%
methane yields 650 Btu per cubic foot (5,857 kcal/cubic meter).
Anaerobic digesters are made out of concrete, steel, brick, or plastic.
They are shaped like silos, troughs, basins or ponds, and may be placed
underground or on the surface. All designs incorporate the same basic
components: a pre-mixing area or tank, a digester vessel(s), a system for
using the biogas, and a system for distributing or spreading the effluent
(the remaining digested material).
There are two basic types of digesters: batch and continuous. Batch-type
digesters are the simplest to build. Their operation consists of loading
the digester with organic materials and allowing it to digest. The
retention time depends on temperature and other factors. Once the
digestion is complete, the effluent is removed and the process is
In a continuous digester, organic material is constantly or regularly fed
into the digester. The material moves through the digester either
mechanically or by the force of the new feed pushing out digested
material. Unlike batch-type digesters, continuous digesters produce biogas
without the interruption of loading material and unloading effluent. They
may be better suited for large-scale operations. There are three types of
continuous digesters: vertical tank systems, horizontal tank or plug-flow
systems, and multiple tank systems. Proper design, operation, and
maintenance of continuous digesters produce a steady and predictable
supply of usable biogas.
Many livestock operations store the manure they produce in waste lagoons,
or ponds. A growing number of these operations are placing floating covers
on their lagoons to capture the biogas. They use it to run an
engine/generator to produce electricity.
The Digestion Process
Anaerobic decomposition is a complex process. It occurs in three basic
stages as the result of the activity of a variety of microorganisms.
Initially, a group of microorganisms converts organic material to a form
that a second group of organisms utilizes to form organic acids.
Methane-producing (methanogenic) anaerobic bacteria utilize these acids
and complete the decomposition process.
A variety of factors affect the rate of digestion and biogas production.
The most important is temperature. Anaerobic bacteria communities can
endure temperatures ranging from below freezing to above 135° Fahrenheit
(F) (57.2° Centigrade [C]), but they thrive best at temperatures of about
98°F (36.7°C) (mesophilic) and 130°F (54.4°C) (thermophilic). Bacteria
activity, and thus biogas production, falls off significantly between
about 103° and 125°F (39.4° and 51.7°C) and gradually from 95° to 32°F
(35° to 0°C).
In the thermophilic range, decomposition and biogas production occur more
rapidly than in the mesophilic range. However, the process is highly
sensitive to disturbances such as changes in feed materials or
temperature. While all anaerobic digesters reduce the viability of weed
seeds and disease-producing (pathogenic) organisms, the higher
temperatures of thermophilic digestion result in more complete
destruction. Although digesters operated in the mesophilic range must be
larger (to accommodate a longer period of decomposition within the tank
[residence time]), the process is less sensitive to upset or change in
To optimize the digestion process, the digester must be kept at a
consistent temperature, as rapid changes will upset bacterial activity. In
most areas of the United States, digestion vessels require some level of
insulation and/or heating. Some installations circulate the coolant from
their biogas-powered engines in or around the digester to keep it warm,
while others burn part of the biogas to heat the digester. In a properly
designed system, heating generally results in an increase in biogas
production during colder periods. The trade-offs in maintaining optimum
digester temperatures to maximize gas production while minimizing expenses
are somewhat complex. Studies on digesters in the north-central areas of
the country indicate that maximum net biogas production can occur in
digesters maintained at temperatures as low as 72°F (22.2°C).
Other factors affect the rate and amount of biogas output. These include
pH, water/solids ratio, carbon/nitrogen ratio, mixing of the digesting
material, the particle size of the material being digested, and retention
time. Pre-sizing and mixing of the feed material for a uniform consistency
allows the bacteria to work more quickly. The pH is self-regulating in
most cases. Bicarbonate of soda can be added to maintain a consistent pH,
for example when too much "green" or material high in nitrogen
content is added. It may be necessary to add water to the feed material if
it is too dry, or if the nitrogen content is very high. A carbon/nitrogen
ratio of 20/1 to 30/1 is best. Occasional mixing or agitation of the
digesting material can aid the digestion process. Antibiotics in livestock
feed have been known to kill the anaerobic bacteria in digesters. Complete
digestion, and retention times, depend on all of the above factors.
Producing and Using Biogas
As long as proper conditions are present, anaerobic bacteria will
continuously produce biogas. Minor fluctuations may occur that reflect the
loading routine. Biogas can be used for heating, cooking, and to operate
an internal combustion engine for mechanical and electric power. For
engine applications, it may be advisable to scrub out hydrogen sulfide (a
highly corrosive and toxic gas). Very large-scale systems/producers may be
able to sell the gas to natural gas companies, but this may require
scrubbing out the carbon dioxide.
Using the Effluent
The material drawn from the digester is called sludge, or effluent. It is
rich in nutrients (ammonia, phosphorus, potassium, and more than a dozen
trace elements) and is an excellent soil conditioner. It can also be used
as a livestock feed additive when dried. Any toxic compounds (pesticides,
etc.) that are in the digester feedstock material may become concentrated
in the effluent. Therefore, it is important to test the effluent before
using it on a large scale.
Anaerobic digester system costs vary widely. Systems can be put together
using off-the-shelf materials. There are also a few companies that build
system components. Sophisticated systems have been designed by
professionals whose major focus is research, not low cost. Factors to
consider when building a digester are cost, size, the local climate, and
the availability and type of organic feedstock material.
In the United States, the availability of inexpensive fossil fuels has
limited the use of digesters solely for biogas production. However, the
waste treatment and odor reduction benefits of controlled anaerobic
digestion are receiving increasing interest, especially for large-scale
livestock operations such as dairies, feedlots, and slaughterhouses. Where
costs are high for sewage, agricultural, or animal waste disposal, and the
effluent has economic value, anaerobic digestion and biogas production can
reduce overall operating costs. Biogas production for generating cost
effective electricity requires manure from more than 150 large animals.
Below-ground, concrete anaerobic digesters have proven to be especially
useful to agricultural communities in parts of the world such as China,
where fossil fuels and electricity are expensive or unavailable. The
primary purpose of these anaerobic digesters is waste (sewage) treatment
and fertilizer production. Biogas production is secondary.
The AgSTAR Program has been very successful in encouraging the development
and adoption of anaerobic digestion technology. Since the establishment of
the program in 1994, the number of operational digester systems has doubled.
This has produced significant environmental and energy benefits, including
methane emission reductions of approximately 124,000 metric tons of carbon
equivalent and annual energy generation of about 30 million kWh. The graph
below shows the historical use of biogas recovery technology for animal
development of anaerobic digesters for livestock manure treatment and energy
production has accelerated at a very fast pace over the past few years.
Factors influencing this market demand include: increased technical
reliability of anaerobic digesters through the deployment of successful
operating systems over the past five years; growing concern of farm owners
about environmental quality; an increasing number of state and federal
programs designed to cost share in the development of these systems; and the
emergence of new state energy policies (such as net metering legislation)
designed to expand growth in reliable renewable energy and green power
the past 2 years alone, the number of operational digester systems has
increased by 30%. For more detailed information on anaerobic digester use in
go to the Guide
to Operational Systems or see the AgSTAR
The process of anaerobic digestion consists of three steps.
The first step is the decomposition (hydrolysis) of plant or animal matter. This step breaks down the organic material to usable-sized molecules such as sugar. The second step is the conversion of decomposed matter to organic acids. And finally, the acids are converted to methane gas.
Process temperature affects the rate of digestion and should be maintained in the mesophillic range (95 to 105 degrees Fahrenheit) with an optimum of 100 degrees F. It is possible to operate in the thermophillic range (135 to 145 degrees F), but the digestion process is subject to upset if not closely monitored.
Many anaerobic digestion technologies are commercially available and have been demonstrated for use with agricultural wastes and for treating municipal and industrial wastewater.
At Royal Farms No. 1 in Tulare, California, hog manure is slurried and sent to a Hypalon-covered lagoon for biogas generation. The collected biogas fuels a 70 kilowatt (kW) engine-generator and a 100 kW engine-generator. The electricity generated on the farm is able to meet monthly electric and heat energy demand.
Given the success of this project, three other swine farms (Sharp Ranch, Fresno and Prison Farm) have also installed floating covers on lagoons. The Knudsen and Sons project in Chico, California, treated wastewater which contained organic matter from fruit crushing and wash down in a covered and lined lagoon. The biogas produce is burned in a boiler. And at Langerwerf Dairy in Durham, California, cow manure is scraped and fed into a plug flow digester. The biogas produced is used to fire an 85 kW gas engine. The engine operates at 35 kW capacity level and drives a generator to produce electricity. Electricity and heat generated is able to offest all dairy energy demand. The system has been in operation since 1982.
Most anaerobic digestion technologies are commercially available. Where unprocessed wastes cause odor and water pollution such as in large dairies, anaerobic digestion reduces the odor and liquid waste disposal problems and produces a biogas fuel that can be used for process heating and/or electricity generation.
describes the anaerobic digestion (AD) process, outlines guidelines for
assessing the feasibility of AD and biogas usage at a swine facility and
provides summary information on AD system performance and reliability.
Digestion Technology Description
AD promotes the
bacterial decomposition of the volatile solids (VS) in animal wastes to
biogas, thereby reducing lagoon loading rates and odor. The primary
component of an AD system is the anaerobic digester, a waste vessel
containing bacteria that digest the organic matter in waste streams under
controlled conditions to produce biogas. As an effluent, AD yields nearly
all of the liquid that is fed to the digester. This remaining fluid
consists of mostly water and is allowed to evaporate from a secondary
lagoon, land-applied for irrigation and fertilizer value or recycled to
flush manure from the swine building to the digester.
The benefits of AD
in the biological oxygen demand of treated effluent by up to 90
percent, reducing the risk for water contamination;
nutrient application control, because up to 70 percent of the nitrogen
in the waste is converted to ammonia, the primary nitrogen constituent
pathogens, viruses, protozoa and other disease-causing organisms in
lagoon water, resulting in improved herd health and possible reduced
water requirements; and
to generate electricity and process heat.
AD takes place in
three steps: hydrolysis, acid formation, and methane generation. During
the first step, hydrolysis, bacterial enzymes break down proteins, fats
and sugars in the waste to simple sugars. During acid formation, bacteria
convert the sugars to acetic acid, carbon dioxide and hydrogen. Then the
bacteria convert the acetic acid to methane and carbon dioxide, and
combine carbon dioxide and hydrogen to form methane and water.
technologies that can be used to collect biogas from swine facilities
mix digesters and
sequencing batch reactor has been used for AD at one swine facility in the
, this technology is considered to be experimental, and thus is not
included in this report. This report focuses on technologies that have
verifiable performance characteristics, namely, covered anaerobic lagoons
and complete mix digesters.
Appendix B provides
contact information that can help producers find AD system
designers/installers, odor control technologies, generators, heating and
cooling equipment, and other information to help manage air and water
quality at hog facilities.
digesters are the simplest AD system. These systems typically consist of
an anaerobic combined storage and treatment lagoon, an anaerobic lagoon
cover, an evaporative pond for the digester effluent, and a gas treatment
and/or energy conversion system. Figure 1 shows a typical schematic for a
floating covered anaerobic lagoon.
Source: EPA. (July 1997). AgStar Handbook: A Manual for Developing Biogas
Systems at Commercial Farms in the
. EPA 430-B-97-015.
. pp. 1-3.
Figure 1 .
Covered anaerobic lagoon digester
digesters typically have a hydraulic retention time (HRT) of 40 to 60
days. The HRT is the amount of time a given volume of waste remains in the
treatment lagoon. A collection pipe leading from the digester carries the
biogas to either a gas treatment system such as a combustion flare, or to
an engine/generator or boiler that uses the biogas to produce electricity
and heat. Following treatment, the digester effluent is often transferred
to an evaporative pond or to a storage lagoon prior to land application.
Climate affects the
feasibility of using covered lagoon digesters to generate electricity.
Engine/generator systems typically do not produce sufficient waste heat to
maintain temperatures high enough in covered lagoon digesters in the
winter to sustain consistently high biogas production rates. Using propane
or natural gas to provide additional heat for the lagoon contents is
typically not an economically viable option. Without that additional heat,
most covered lagoon digesters produce less biogas in colder temperatures,
and little or no gas below 39 FACE= "Symbol">° F. As a
result, covered lagoon digesters are most appropriate for use in warm
climates if the biogas is to be used for energy or heating purposes.
digester systems consist of a mix tank, a complete mix digester and a
secondary storage or evaporative pond. The mix tank is either an
aboveground tank or concrete in-ground tank that is fed regularly from
underfloor waste storage below the animal feedlot. Waste is stirred in the
mix tank to prevent solids from settling in the waste prior to being fed
to the digester. The complete mix digester is essentially a
constant-volume aboveground tank or in-ground covered lagoon that is fed
daily from the mix tank. Complete mix digesters with in-ground lagoons
often employ covers similar to those used in covered lagoon digesters. In
the digester, a mix pump circulates waste material slowly around the
heater to maintain a uniform temperature. Hot water from an
engine/generator cogeneration water jacket or boiler is used to heat the
digester. A cylindrical aboveground tank, such as that shown in Figure 2,
optimizes biogas production, but is more capital intensive than in-ground
tanks. The only operating AD system in
that recovers methane for energy use is a complete mix digester, located
at Colorado Pork LLC near
EPA. (February 1997). AgStar Technical Series: Complete Mix Digesters –
A Methane Recovery
Option for All Climates. EPA 430-F-97-004.
Figure 2 . Complete
mix digester schematic
digesters have an HRT of 15 to 20 days, which means that complete mix
digesters can reduce the overall lagoon volume required for waste storage
and treatment. This makes complete mix digesters comparable to covered
lagoon digesters in cost, despite the increased complexity of stirring,
mixing and plumbing components. In addition, biogas production rates, and
therefore heat and electricity production, are greater and more consistent
than for covered lagoons. This can help reduce system payback periods
compared to covered lagoon systems. Like covered lagoon systems, digester
effluent from complete mix digesters is frequently stored in evaporative
ponds or storage lagoons.
provides guidelines for conducting a preliminary assessment of the
feasibility of using AD at a swine facility. Although AD system
requirements will vary depending on the application and system design,
there are some rule-of-thumb measures that should be noted when assessing
the feasibility of AD at a given location. For AD to potentially be
technically feasible and cost-effective, a swine facility should:
house at least 2,000 animals with a total live animal weight of at
least 110,000 pounds,
no more than 20 percent variation in animal population throughout the
waste at one central location such as an underfloor pit,
waste daily or every other day, or can convert to an equivalent
manure free of large amounts of bedding or other foreign materials,
some manure storage capability to maintain a steady digester feedstock
If the above
characteristics are present, the facility is a possible candidate for AD.
Many pre-existing waste storage and treatment lagoons are too large to
practically or cost-effectively employ covers over their entire area.
Partial covers may be an option to recover methane from these older
systems, as an alternative to installing a completely new storage and
treatment lagoon system.
If energy recovery
is to be employed, methane production and gas quality should be considered
and compared to energy requirements at the facility. Daily biogas
production at installed farm-based anaerobic digesters in the
varies from 24,000 to 75,000 cubic feet, or an energy equivalent of 13 to
42 million British thermal units (Btu) (assuming 55 percent methane
content for biogas). Covered lagoon digesters and complete mix digesters
differ in their methane production characteristics, and energy conversion
systems that rely on methane from anaerobic digesters should be chosen
according to the end-use objective for the system. Complete mix digesters
can produce heat and electricity at a constant rate throughout the year
because heat recovery can be used to heat the digesters in the winter.
Covered lagoon digesters can consistently produce biogas only in months
when the temperature exceeds 39 degrees Fahrenheit.
Facilities that are
located south of the line of climate limitation in Figure 3 are usually
warm enough for cost-effective energy recovery from covered lagoon
digesters. In most cases, facilities north of the climate line in Figure 3
are too cold for cost-effective energy recovery from covered lagoon
digesters. Complete mix digesters can be used in cold or warm climates. If
odor control is the only objective, either covered lagoon or complete mix
digesters may be used, but odor control will be less effective in the
winter for covered lagoon digesters south of the line of climate
limitation in Figure 3. In general, complete mix digesters are the most
appropriate choice for use in
Source: EPA. (July 1997). AgStar Handbook: A Manual for Developing Biogas
at Commercial Farms in the
. EPA 430-B-97-015. pp. 4-12.
Figure 3 . Line of
climate limitation for biogas energy recovery
Table 2 shows which
digesters are appropriate for the waste collection strategies at covered
swine facilities. Complete mix digesters can operate with a waste total
solids (TS) percentage between 3 and 10 percent, while covered lagoon
digesters can use waste with a TS percentage less than 2 percent.
2 . Matching a digester to existing waste collection practices
Warm or cold
Warm or cold
Warm or cold
Source – Adapted
from: EPA. (July 1997). AgStar Handbook: A Manual for Developing Biogas
Systems at Commercial
Farms in the
. EPA 430-B-97-015. pp. 4-15.
Appendix C describes
each of the various waste collection technologies listed in Table 2.
discusses some of the biogas utilization options that are available for
use with AD. Electricity generation with waste heat recovery
(cogeneration) and direct combustion and use in equipment that normally
uses propane or natural gas are the two primary options for biogas
utilization. Electricity generated using biogas can be generated for
on-farm use or for sale to the electric power grid if an economically
attractive power purchase agreement can be negotiated through the local
utility or rural electric cooperative. Direct combustion allows the gas to
be used in existing equipment that normally uses propane or natural gas
such as boilers or forced air furnaces with minor equipment modifications.
Combustion is usually a seasonal use for biogas, as most boiler and
furnace applications are only required during the winter. The EPA FarmWare
manual describes some characteristics of engine/generator and direct
combustion systems that can be used with biogas. The following subsections
draw from the FarmWare manual to provide some basic information about the
use of these systems at covered swine facilities and other farm
electricity generation systems that use biogas typically consist of an
internal combustion (IC) engine, a generator, a control system and an
optional heat recovery system.
IC engines designed
to burn propane or natural gas are easily converted to burn biogas by
adjusting carburation and ignition systems. Such engines are available in
nearly any capacity, but the most successful varieties are industrial
engines that are designed to work with wellhead natural gas. A
biogas-fueled engine will normally convert 18 to 25 percent of the biogas
Btu value to electricity.
Two types of
generators are used on farms: induction generators and synchronous
generators. Induction generators operate in parallel with the utility and
cannot operate as a stand-alone power source. Induction generators derive
their phase, frequency and voltage from the utility. Synchronous
generators operate as an isolated system or in parallel to the utility,
and require more sophisticated intertie systems to match output to utility
phase, frequency and voltage.
Control systems are
required to protect the engine and the utility. Control packages are
available that can shut the engine off due to mechanical problems, utility
power outage or utility voltage and frequency fluctuations, or in the
event that excess power is generated that the utility will not accept.
Generators that operate in parallel with the utility system, such as
induction generators, require an intertie system with safety relays to
shut off the engine and disconnect from the utility in the event of a
problem. Intertie negotiations with a utility for induction generators are
typically much easier than for a synchronous generator, due to the level
of control the utility has over the characteristics of power entering the
grid from an induction generator. The primary advantage of a synchronous
generator is its ability to act as a stand-alone power source. However, if
operated as an isolated system, a synchronous generator must be oversized
to meet the highest electrical demand, while operating less efficiently at
average or partial loads. Due to the system size and more complicated
control requirements, a synchronous generator operating as an isolated
system is typically more expensive than an induction generator.
reject approximately 75 to 82 percent of the energy input as waste heat.
This waste heat can be used to heat the digester and/or provide water or
space heat to the facility. Commercial heat exchangers can recover waste
heat from the engine water cooling system and the engine exhaust,
recovering up to 7,000 Btu/hour for each kW of generator load. Waste heat
recovery increases the energy efficiency of the system to 40 to 50
Ongoing research and
development is focusing on the use of microturbines and fuel cells for
converting biogas to electricity. Microturbines are high-speed,
small-scale (typically less than 100 kW) gas-driven turbine systems that
produce electricity efficiently, have low emissions and require little
maintenance. Reflective Energies in
in partnership with Capstone Microturbine Corporation is working on
developing the Flex-Microturbine, a power generation technology that can
use biogas from animal waste, landfill gas and biomass gasification as its
fuel source. Fuel cells are an emerging technology that operate, in
principle, like a battery, but do not run out of charge. Instead, fuel
cells equipped with a fuel reformer can use any type of hydrocarbon fuel,
and run continuously as long as fuel is available. Fuel cells can convert
fuel to electricity at efficiencies close to 40 percent, compared to 30
percent for the most efficient engine. In addition, fuel cell emissions
include heat, some of which can be recovered for other applications,
water, and carbon dioxide.
The Department of
Energy’s WRBEP funded a project in fiscal year 2000 in
San Luis Obispo
that will demonstrate electricity generation from methane using a
prototype microturbine at a 350-cow farm. The project will be using a 25
kW Capstone microturbine prototype to generate electricity at the
’s demonstration farm.
Direct combustion of
biogas on-site in a boiler or forced air furnace can provide seasonal heat
to nurseries, farrowing rooms and other facilities at a swine facility. A
cast iron natural gas boiler can be used for most farm boiler
applications. The air-fuel mixture will require adjustment and burner jets
will need to be enlarged for use with low-Btu gas. Cast iron boilers are
available in many sizes, from 45,000 Btu/hour and up. Untreated biogas may
be used, but all metal surfaces of the boiler housing should be painted to
prevent corrosion. Flame tube boilers with heavy gauge flame tubes may be
used if the exhaust temperature is maintained above 300 FACE=
"Symbol">° F to prevent condensation. Forced air furnaces
can be used in place of direct fire room heaters, but biogas must be
treated to remove hydrogen sulfide because of potential corrosion problems
in metal ductwork.
Performance and Benefits of AD
There are several
measures of waste management system performance that are relevant for
producers considering the use of AD. These include:
AD is the only waste
management strategy available that provides the option to recover methane
for energy production.
The APCD has
determined that the minimum standard for compliance with odor control
regulations for waste vessels and impoundments is an 80 percent reduction
in all odor-causing gases, including hydrogen sulfide, ammonia and
volatile organic compounds from waste vessels or impoundments. Table 3
compares the effectiveness of some of the odor control methods being
implemented at covered swine facilities in
. Lagoon covers and AD are among the most effective means of reducing
odors from waste storage and treatment systems. However, several
strategies may be combined to increase the effectiveness of individual
odor control strategies at a facility. As an example, feed additives can
be used in conjunction with biofilters, surface aeration or solids
separation to increase overall odor control from waste storage and
treatment lagoons. In addition, any lagoon odor control technology should
be accompanied by an overall odor management program using best management
practices as described in Appendix D.
3 . Odor control effectiveness of management strategies for
odorous gas emissions reduction
hogs (3:1 water to feed)
sulfur-containing amino acids
fiber (soybeans, hulls to diet)
of waste upon land application
(land application odors only)
to 100 for well-managed systems
Kirk and Jessica Davis. (February 1999). Innovations in odor management
. Agricultural and Resource Policy Report. APR-99-02.
In addition to
regulating odors from waste lagoons, the new odor control regulations have
requirements for waste that is applied to agricultural land. The new
regulations for waste treatment at covered swine facilities require that
waste applied to agricultural land and not injected be treated to remove
at least 65 percent of the TS and over 90 percent of the total volatile
fatty acids or 60 percent of total VS. If not treated, waste applied to
agricultural land must be injected or knifed into the soil upon
application. Land application is not permitted between November 1 and
February 28. Of the waste management strategies in Table 3, four will help
reduce the TS and VS content prior to land application.
Wet feeding can
reduce the TS and VS by a value equal to the dilution rate of the feed
(i.e., 3:1 ratio of water to feed). However, introducing this type of
feeding system increases water requirements and may increase required
anaerobic lagoon volumes. Solids separation can reduce TS by 30 to 45
percent. Solids separation methods include screen separators, mechanical
presses, settling tanks, settling basins, vacuum filters and many other
means. An efficient AD installation will reduce the TS percentage by up to
76 percent and VS by up to 90 percent. Of the above technologies, AD with
covered anaerobic lagoons is the only one the APCD considers a proven
technology because of their odor control effectiveness. Therefore, unlike
the other options above, covered anaerobic digesters do not have to meet
the additional testing requirements for technologies that the APCD
Composting may or
may not meet the TS requirement because it often involves the addition of
a bulking agent to increase TS to optimize waste decomposition. However,
composting can be effective at controlling odors and reducing pathogens.
The APCD is presently reviewing the compliance status of one facility that
uses composting. Composting has applications besides manure treatment for
livestock facilities. The Colorado Governor’s Office of Energy
Management and Conservation is currently supporting the demonstration of
composting technology for hog mortality disposal at a hog farm in
In an AD system,
most of the organic nitrogen (N) from the digester is converted to
ammonium, an easily manageable fertilizer with slow release properties
when compared to mineralized fertilizers. This is an advantage over
anaerobic lagoons alone. Organic N in the form of protein and urea is
mineralized in soil solution after land application. This mineralized N
can pose a groundwater problem when land-applied because mineralized N can
be converted to nitrates and leach into groundwater in the spring and fall
when plant uptake of N is low.
A disadvantage of
reducing the nutrient content of lagoon effluent via AD is the loss of the
value of nutrients. Reducing the use of lagoon effluent as fertilizer
increases the need for industrial fertilizers, the manufacture and
transportation of which uses significant quantities of petroleum. However,
this loss is balanced by the benefits of increased control farmers have
over the nutrient content of effluent used for irrigation purposes.
is a key concern for swine producers that are considering AD with energy
recovery as an objective. AD systems first began to be used extensively
after World War II in
when energy supplies were reduced. Today there are over 600 digesters in
alone. Farm-based anaerobic digesters are the most common application of
AD technology worldwide. In the
, livestock producers have less experience working with anaerobic
digesters, with a total of approximately 160 digesters either planned or
installed in 1998. Of these, 36 employ technology that is suitable for use
at swine facilities.
A recent survey of
anaerobic digesters yielded mixed results for system reliability (Table
4). At farms across the
, the percentage of installed digesters that are not operating is nearly
46 percent. However, one encouraging note is that the reliability of
digesters constructed since 1984 is much greater than for those
constructed between 1972 and 1984.
4 . Status of farm-based digesters at swine facilities in the
Source: Lusk, Phil
(September 1998). Methane Recovery from Animal Manures: the Current
Opportunities Casebook. NREL/SR-25145. NREL. Golden, CO. pp. 1-2.
The most common
reasons that systems are not operating include poor design and
installation and poor equipment specification. The lessons learned that
should be kept in mind for future systems include the need to select
qualified contractors and the fact that amortizing the cost of appropriate
equipment is less costly than a system failure. The improved reliability
of newer systems and increased understanding of the biological systems
that operate in an anaerobic digester suggest that the reliability of
systems will continue to improve as long as the lessons of past system
failures are heeded.
What is BioMethane?
BioMethane is a renewable energy/fuel, with properties similar to natural
gas, produced from "biomass." Unlike natural gas, BioMethane is a renewable energy.
The cost of producing
BioMethane, after installation of the
BioMass Gasification equipment used to produce BioMethane (the process of
making BioMethane is called "BioMethanation") is called is
Again, unlike the price of natural gas, which has been around $6.00/mmbtu
for the past year.
Gasification and BioMethanation Technology
The production and disposal of large quantities of organic and biodegradable waste without adequate
or proper treatment results in widespread environmental pollution. Some waste streams can be treated by conventional methods like aeration. Compared to the aerobic method,
the use of anaerobic digesters in processing these waste streams provides
greater economic and environmental benefits and advantages.
As previously stated,
Biomethanation is the process of conversion of organic matter in the waste (liquid or solid) to
BioMethane (sometimes referred to as "BioGas) and manure by microbial action in the absence of
air, known as "anaerobic digestion."
Conventional digesters such as sludge digesters and anaerobic CSTR (Continuous Stirred Tank
Reactors) have been used for many decades in sewage treatment plants for stabilizing the activated sludge and sewage solids.
Interest in BioMethanation as an
economic, environmental and energy-saving waste treatment continues to gain
greater interest world-wide and has led to the development of a range of anaerobic reactor designs. These high-rate,
high-efficiency anaerobic digesters are also referred to as "retained biomass reactors" since they are based on the concept of retaining viable biomass by sludge immobilization.
Biomass Gasification and the Production of BioMethane
Biomass is a renewable energy resource which includes a wide variety if organic resources. A few of these include wood, agricultural residue/waste, and animal manure.
Biomass Gasification is the process in which BioMethane is produced in the BioMass Gasification process. The BioMethane is then used like any other fuel, such as natural gas, which is not a
Historically, biomass use has been characterized by low btu and low efficiencies. However, today biomass gasification is gaining world-wide recognition and favor due to the economic and environmental benefits. In terms of economic benefits, the cost of the
BioMethane is essentially free, after the cost of the equipment is installed.
BioMethane, probably the most important and efficient energy-conversion technology for a wide variety of biomass fuels.
The large-scale deployment of efficient technology along with interventions to enhance the sustainable supply of biomass fuels can transform the energy supply situation in rural areas.
It has the potential to become the growth engine for rural development in the country.
Principles of Biomass Gasification
Biomass fuels such as firewood and agriculture-generated residues and wastes
are generally organic. They contain carbon, hydrogen, and oxygen along with some moisture. Under controlled conditions, characterized by low oxygen supply and high temperatures, most biomass materials can be converted into a gaseous fuel known as producer gas, which consists of carbon monoxide, hydrogen,
carbon dioxide, methane and nitrogen. This thermo-chemical conversion of solid biomass into gaseous fuel is called biomass gasification. The producer gas so produced has low a calorific
value (1000-1200 Kcal/Nm3), but can be burned with a high efficiency and a good degree of control without emitting smoke. Each kilogram of air-dry biomass (10% moisture content) yields
about 2.5 Nm3 of producer gas. In energy terms, the conversion efficiency of the gasification process is in the range of 60%-70%.
Multiple Advantages of Biomass Gasification
Conversion of solid biomass into combustible gas has all the advantages associated with using gaseous and liquid fuels such as clean combustion, compact burning equipment,
high thermal efficiency and a good degree of control. In locations, where biomass is already available at reasonable low prices (e.g. rice mills) or in industries using fuel wood,
gasifier systems offer definite economic advantages. Biomass gasification technology is also environment-friendly, because of the firewood savings and reduction in CO2 emissions.
Biomass gasification technology has the potential to replace diesel and other petroleum products in several applications, foreign exchange.
Applications for Biomass Gasification
Thermal applications: cooking, water boiling, steam generation, drying etc.
Motive power applications: Using producer gas as a fuel in IC engines for applications such as water pumping
Electricity generation: Using producer gas in dual-fuel mode in diesel engines/as the only fuel in spark ignition engines/in gas turbines.
Publicly Owned Treatment Works ("POTW's")
or Wastewater Treatment Systems
More and more,
cities, counties and
municipalities are faced with greater environmental compliance issues relating
to their municipally-owned landfills, Publicly Owned Treatment Works ("POTW's")
or Wastewater Treatment Systems. A city's landfill and/or POTW provides an excellent
opportunity for cities to reduce their emissions as well as provide an
additional revenue stream. These facilities may have valuable gases
that our company recovers and pipes to one of our clean,
environmentally-friendly cogeneration or trigeneration energy
systems. We solve a city's environmental liabilities (air emissions)
and provide a new cash flow simultaneously. We offer turn-key
solutions for cities that includes the preliminary feasibility analysis,
engineering and design, project management, permitting and
commissioning. We provide very attractive financing packages for
cities that does not add to a city's liability, yet provides a valuable
new revenue stream. And, we are also able to offer a turn-key
solution for qualified municipalities that includes our company owning,
operating and maintaining the onsite power and energy plant.
At the heart of
the system is a (Bio) Methane Gas Recovery system similar those used in Flare Gas Recovery or Vapor Recovery
Units. Methane Gas Recovery, Flare Gas Recovery,
Vapor Recovery, Waste to Energy and Vapor Recovery Units all recover valuable
"waste" or vented fuels that can be used to provide fuel for an
onsite power generation plant. Our waste-to-energy and waste to fuel
systems significantly or entirely, reduces your facility's emissions (such
SOx, H2S, CO
, CO2 and other Hazardous Air Pollutants/Greenhouse Gases) and convert
these valuable emissions from an environmental problem into a new cash
revenue stream and profit center.
and vapor recovery units can be located in hundreds of applications and
locations. At a landfill, Wastewaster Treatment System (or Publicly Owned
Treatment Works - "POTW") gases from the facility can be
captured from the anaerobic digesters, and manifolded/piped to one of our
onsite power generation plants, and make, essentially, "free"
electricity for your facility's use. These
that are generated from municipally owned landfills or wastewater
treatment plants have low btu content or heating values, ranging around
550-650 btu's. This makes them
unsuitable for use in natural gas applications. When burned as fuel to
generate electricity, however, these gases become a valuable source of
"renewable" power and energy for the facility's use or resale to
the electric grid.
heat (steam and/or hot water) is required, we will incorporate our
cogeneration or trigeneration system into the project and provide some, or
all, of your hot water/steam requirements. Similarly, at crude oil
refineries, gas processing plants, exploration and production sites, and
gasoline storage/tank farm site, we convert your facility's "waste
fuel" and environmental liabilities into profitable,
Our Methane Gas
Recovery systems are designed and engineered for
these specific applications. It is important to note that there are
many internal combustion engines or combustion turbines that are NOT
suited for these applications. Our systems are engineered precisely
for your facility's application, and our engineers know the engines and
turbines that will work as well as those that don't. More
importantly, we are vendor and supplier neutral! Our only
concerns are for the optimum system solution
for your company, and we look past brand names and sales propaganda to
determine the optimum system, which may incorporate either one or more;
gas engine genset(s) or gas turbine genset(s), in cogeneration or
trigeneration mode - in trigeneration mode, we incorporate absorption
chillers to make chilled water for process or air-conditioning, fuel
gas conditioning equipment and gas compressor(s).
systems includes design, engineering, permitting, project management,
commissioning, as well as financing for our qualified customers.
Additionally, we may be interested in owning and operating the flare gas
recovery or vapor recovery units. For these applications, there is no
investment required from the customer.
information, please provide us with the following information about the
flare gas or vapor:
Type of gas
being flared or vented (methane, bio-gas, digester, landfill, etc.).
Fuel/Gas analysis which provides us with the btu's (heating value) and
the composition of the gas and its' impurities such as methane (and
the percentage of methane), soloxanes, carbon dioxide, hydrogen,
hydrogen sulfide, and any other hydrocarbons.
of gas available, from all sources, at the facility.
lagoons are perhaps the most trouble free, low maintenance systems
available for treatment of animal waste. This is particularly true in the
By Leland M. Seale,
Environmental Engineer, USDA-NRCS
of the above information from the Department of Energy and U.S. Department
of Agriculture with permission, and our thanks.