Note: Descriptions are shown in the official language in which they were submitted.
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REGENERATIVE THERMAL OXIDIZER FOR DIE REDUCTION OR ELIMINATION
OF SUPPLEMENTAL FUEL GAS CONSUMPTION'
Field of the Invention
The invention broadly relates to boas applications and, more particularly, to
a
regenerative thermal oxidizer for the reduction or elimination of supplemental
fuel gas
consumption..
Background of the Invention
Biogas refers to a gaseous fuel produced by the biological breakdown of
organic matter
in the absence of oxygen. it is produced by the anaerobic digestion or
fermentation of
biodegradable materials such as biomass, manure, sewage, municipal waste,
green waste, plant
material and crops. .Biogas primarily comprises methane and carbon dioxide,
and may contain
small amounts of hydrogen sulphide:, moisture and .siloxanes.
The gases methane, hydrogen and carbon monoxide can be combusted or oxidized
with
oxygen. This energy release allows biogas to be used. as a fuel. Biogas can be
used as a fuel for
any heating purpose. It can also be produced by anaerobic digesters where it
is typically used in.
a gas engine to convert the chemical energy of the gas into electricity and
heat. Anaerobic
digestion is a series of processes in which microorganisms break down
biodegradable material in.
the absence of oxygen, also used for industrial or domestic purposes to manage
waste and/or to
release energy.
The digestion process begins with bacterial hydrolysis of the input materials
in order to.
break down insoluble organic polymers such as carbohydrates and make them
available for other
bacteria. Acidogenic bacteria then convert the sugars and amino acids into
carbon dioxide,
hydrogen, ammonia, and organic acids. These bacteria then convert these
resulting organic acids
into acetic acid, along with additional ammonia, hyd.mgen, and carbon dioxide.
Finally,
methanog.ens convert these .products to methane and carbon dioxide.
Anaerobic digesters can use a multitude of .fied stocks for the production of
methane rich
bio-gas including but not limited to purpose-grown energy crops such as maize.
Landfills also
produce methane rich bio-gas through the anaerobic digestion process. As part
of an integrated
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waste management system, this bio-gas may be collected and processed for
beneficial use while
simultaneously reducing greenhouse gas emissions into the atmosphere.
aerobic digestion is widely used as a source of renewable energy. The process
produces a biogas that. can be used directly as cooking fuel, in combined heat
and power gas
engines or upgraded to natural gas quality biomethane. The utilization of
biogas as a fuel helps to
replace fossil fuels. The nuttientarich digestate and/or Leachate that is also
produced can he used
as fertilizer.
The technical expertise required to maintain industrial scale anaerobic
digesters coupled
with high capital costs and low process efficiencies have limited the level of
its industrial
application as a. waste treatment technology. As a result, it is imperative
that anaerobic digesters
and landfill gas treatment plants operate at the 'highest possible efficiency.
Summary of the Invention
The present invention provides a regenerative thermal oxidizer for the
reduction or
elimination of supplemental filel gas consumption as well as allowing for the
maximization of
the product methane recovery.
One embodiment of the invention is directed toward a regenerative thermal
oxidizer
system, comprising: a recuperative heat exchanger that receives process
exhaust gas from an.
anaerobic digestion process and boosts an inlet gas temperature of the gas to
a second gas
temperature; and a regenerative thermal oxidizer that receives the gas from
the heat exchanger,
further heats the gas to the point of auto ignition or combustion .to a third
gas temperature, and
feeds the gas back through the recuperative heat exchanger, .which recoups
.heat from the gas
such that the gas exits the heat exchanger at a fourth gas temperature,
In some implementations of the regenerative thermal oxidizer system, the inlet
temperature of the gas is about 90T, the second gas temperature is about 400
'I', the third. gas
temperature is about 1200 F, and the gas exhaust. temperature is about 500 F.
Boosting the inlet
temperature of the gas to the second gas temperature allows the thermal
oxidizer to operate
without the use of additional .fuel,
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Another embodiment of the invention is directed toward a regenerative thermal
oxidizer
system similar to the above embodiment, but designed to receive process
exhaust gas from a
landfill gas system.
A further embodiment of the invention is directed toward a method for using a
regenerative thermal oxidizer, comprising: receiving at a recuperative heat
exchanger process
exhaust from an anaerobic digestion process and/or a landfill gas system;
boosting an inlet gas
temperature of the exhaust gas to a second gas temperature; receiving at a
regenerative thermal
oxidizer the exhaust .from the heat exchanger; further heating the gas to a
third gas temperature;
feeding the gas back through the recuperative heat exchanger; and recouping,
heat from the gas
such that the gas exits the heat exchanger at a fourth gas temperature.
According to some embodiments of the above method, the inlet temperature of
the gas is
about 90 F, the second gas temperature is about 400 "F, the third gas
temperature is about 1200
'V, and the gas exhaust temperature is about 500 F. Boosting the inlet
temperature of the gas to
the second gas temperature may comprise allowing the thermal oxidizer to
operate without the
.use of additional fuel.
Brief Description of the Drawings
Figure 1. is a flow diagram illustrating the stages of an exemplary anaerobic
digestion
system.
Figure 2 is a diagram illustrating .the stages of an exemplary landfill gas
system.
Figure 3 is a diagram illustrating the use of a regenerative thermal oxidizer
system in
accordance with an embodiment of the invention.
Detailed Description
In the following paragraphs, the present invention will be described in detail
by way of
example .with reference to the attached drawings. Throughout this description,
the preferred.
embodiment and examples shown should be considered as exemplars, rather than
as limitations
on the present invention. As used herein, the "present invention" refers to
any one of the
embodiments of the invention .described herein, and any equivalents.
Furthermore, reference to
various feature(s) of the "present invention" throughout this document, does
not mean that all
claimed embodiments or methods must include the referenced feature(s)..
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Biogas is a renewable energy composed primarily of methane resulting from the
natural
decomposition of organic waste by anaerobic bacteria. Similar to natural gas,
methane captured
by a biogas system can be used to provide heat, electrical power or
transportation biofuel.
Biogas extraction can be used to: (i) produce green and renewable energy; (ii)
reduce pollution
and greenhouse gases; reduce waste odors and pathogens; and transform waste
into valuable
bio-fertil izer.
Fermentation, or anaerobic digestion, is the most common process that breaks
down the
organic waste. The organic waste may then be oxidized, thereby creating
energy. Various types
of organic materials include, but are not limited to: (i) biomass, (ii)
landfill waste, (iii) sewage,
(iv) manure, and (v) plant material. The most common gases produced are
.methane and carbon
dioxide. Other gases that can be formed include hydrogen, nitrogen, and carbon
monoxide.
Methane, hydrogen, and carbon monoxide can be combusted to create beat and
electricity,
When biogas is created from existing waste streams, it reduces odors and
methane emissions and
creates two renewable resources. Methane is a potent greenhouse gas that
contributes to global
climate change. It is expected that a landfill gas energy project will capture
about 60% to 90%
of the methane emitted from the landfill, depending on system design and
effectiveness.
There are two primary methods of recovering biogas for use as energy, namely:
(i) by
creating an anaerobic digestion system to process waste, most commonly manure
or other wet
biomass, and (ii) by recovering natural biogas production formed in existing
landfills. Once
recovered, biogas can be converted to energy using a number of methods.
Figure 1 is a flow diagram illustrating the stages of an exemplary anaerobic
digestion
system 100. Specifically, the an anaerobic digestion system. 100 comprises a
manure collection
system 110, a manure handling system 120, an anaerobic digester 130, a biogas
handling system
140, gas use devices 150, an effluent storage 160. In addition, at least one
flare 170 may be used
to burn excess gas. Digester products 180 may be used for bedding, potting
soil, land
applications, etc. More particularly, manure collection system 110 is used to
gather manure and
transport it to the anaerobic digester 130. In some cases, existing
liquidislurry manure
management systems can be adapted to deliver manure to the anaerobic digester
1.30. The
anaerobic digester 130 may be designed to stabilize manure and optimize the
production of
methane. A storage facility for digester effluent, or waste matter, is also
required.
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With further reference to Figure I, the anaerobic digester 130 outputs biogas
into the
biogas handling system .140. The biogas may contain approximately 60% methane
and 40%
carbon dioxide. It is collected, treated, and piped to a gas use device 150.
By way of example,
the biogas can then be upgraded to natural gas pipeline quality. It may also
be used to generate
electricity, as a boiler fuel for space or water heating, or for a variety of
other .uses. At least one
flare 1.70 is also installed to destroy extra gas and as a back-up mechanism
for the .primary gas
use device 160.
The anaerobic digester 1$0 may be made out of concreteõ steel, brick, or
plastic.
Additionally, the digester 130 includes a tank for .pre-mixing the waste and a
digester vessel. In
1.0 sonic embodiments, the anaerobic digester 130 may comprise a batch
digesters or a continuous
digester. A batch digester is loaded with organic materials, which are allowed
to digest therein.
The retention time depends on temperature and other -ffictors. Once the
digestion is complete, the
effluent is removed and the process is repeated.
In further embodiments, the anaerobic digester 1.30 may comprise a continuous
digester,
wherein organic material is constantly or regularly fed into the digester, and
.wherein the material
moves through the digester either mechanically or by the force of the new
feed. Unlike 'batch-
type digesters, continuous digesters produce biogas without the interruption
of loading material
and unloading effluent. Various types of continuous digesters include
.vertical tank systems,
horizontal tank or plug-flow systems, and multiple tank. systems.
Anaerobic digestion also occurs naturally underground in landfills, wherein
the waste is
covered and compressed by the weight of the material that is deposited above.
This material
prevents oxygen exposure, thereby allowing chemical reactions and microbes to
act upon the
waste. This encourages an uncontrolled process of biomass decay. The rate of
production is
affected by waste composition and landfill geometry. Landfill gas may comprise
about 40% to
60% methane, and about 40% to 60% carbon dioxide.
Figure 2 is a diagram illustrating the stages of an exemplary landfill gas
system 200
including landfill 210, landfill gas wells 220 for active gas collection,
landfill gas wellhead 230,
landfill gas processing and treatment plant 240, and at least one landfill gas
flare 250, Landfill
gas is extracted from landfill 210 using a series of wells 220 and a
blower/flare system. The
landfill gas system 200 directs the collected gas to landfill gas processing
and treatment .plant
240, where it is processed and treated.
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The gas clean-up process creates two streams, namely the product stream and an
off spec
gas that needs to be destroyedõk thermal oxidizer may be employed to burn this
exhaust gas.
However, a typical thermal oxidizer is limited by the lower flammability
limits of combustion of
a gas.
The desired product gas methane recovery of current clean-up technologies is
limited by
this requirement. Typical volumetric percent values of methane .required for
combustion in a
standard thermal oxidizer range from 8-15% vol. Hence, a system with a higher
product
methane recovery would not have enough heat/methane content in the exhaust gas
to allow for
stable combustion unless supplemental fuel was used.
According to embodiments of the invention, a "regenerative thermal oxidizer"
preheats
the inlet gas with its own hot exhaust gas, thereby bootstrapping the
combustion process. This
increase in inlet gas temperature to the thermal oxidizer decreases the
flammability limit
allowing for the destruction of a. lower quality methane stream (le., allowing
for a higher
product methane recovery to the product gas stream). Typical ranges of methane
required for
stable combustion in a regenerative thermal oxidizer can be as low as 0.5%
vol., thus allowing
for a higher methane recovery to be possible for the gas clean-up system
Figure 3 is a diagram illustrating .the use of a regenerative thermal oxidizer
system 300 in
accordance with an embodiment of the invention. The regenerative thermal
oxidizer system 300
is configured to be used as part of an anaerobic digestion system 100 such as
disclosed with
respect to Figure 1 and a landfill gas system 200 such as disclosed with
respect to Figure 2. In
particular, process exhaust gas 310 from the anaerobic digestion system 100 or
landfill gas
system 200 is fed into a recuperative heat exchanger 320, which boosts the
inlet temperature and
feeds the gas into a regenerative thermal oxidizer 330. This process exhaust
gas 310 is excess
gas, e.g., from an anaerobic digestion system .100 or a landfill gas system
200, that would
normally be destroyed using a flare or standard non-regenerative thermal
oxidizer.
With further reference to figure 3, the thermal oxidizer 330 heats the gas
such that it exits
the thermal oxidizer at a much higher temperature and is fed back through the
recuperative heat
exchanger 320, which recoups the excess heat and lowers the temperature of the
gas. By way of
example, the process exhaust gas 3.10 may be fed into the heat exchanger 320
at a temperature of
about 90 1 and is boosted to about 400 T before entering the regenerative
thermal oxidizer 330.
In this example, the regenerative thermal oxidizer 330 may raise the gas
temperature to about
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1200 "F and the heat exchanger then reduces the temperature to about 500 'F.
By boostin!s,, the
gas temperature, the -recuperative heat exchanger 320 allows the thermal
oxidizer 330 to operate
without the use of additional fuel.
It is desirable that plant processes such as anaerobic digestion systems 1.00
and landfill
gas systems 200 operate at the highest efficiency possible. in such processes,
this requires very
low levels of methane loss in the exhaust gas. However, while a low methane
exhaust gas
stream is great for plant efficiency, it is not good for the thermal oxidizer.
In particular, if the
plant is too efficient, a typical thermal oxidizer system may require the
addition of supplemental.
fuel to maintain stable combustion, thereby reducing he overall plant
efficiency.
A typical thermal oxidizer requires approximately CH4 to maintain stable
combustion. By contrast, the regenerative thermal oxidizer 330 in the
embodiment of Figure 3
can operate with as low as 0.5% C114, thus allowing for a substantial increase
in plant processing.
efficiency. This is achieved by allowing the incoming reactants to be reheated
with the effluent
hot gas from the thermal oxidizer exhaust gas stream. Accordingly, the
regenerative -thermal
oxidizer 330 significantly increases plant efficiency by eliminating the
penalty associated with
conventional thermal oxidizers.
A Wolter embodiment of the invention is directed toward a method -for using a
regenerative thermal oxidizer, comprising: receiving at a recuperative heat
exchanger process
exhaust gas front an anaerobic digestion system or a landfill gas system;
boosting an inlet
temperature of the gas to a second exhaust temperature; receiving at a
regenerative thermal
oxidizer the gas from the heat exchanger; further heating/combusting the gas
to a third exhaust
temperature; feeding the gas back through the recuperative heat exchanger; and
recouping heat
from the gas such that the gas exits the heat exchanger at a fourth gas
temperature.
According to some embodiments of the above method, the inlet temperature of
the
exhaust .gas is about 90 "F, the second gas temperature is about 400 'I', the
third gas temperature
is about 1200 riF, and the fourth gas temperature is about 500 'F. Boosting
the inlet temperature
of the gas to the second gas temperature may comprise allowing the thermal
oxidizer to operate
without the use of additional fuel.
One skilled in the art will appreciate that the present invention can be
practiced by other
10 than the various embodiments and preferred embodiments, which are
presented in this
description for purposes of illustration and not of limitation, and the
present invention is limited
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only by the claims that f011ow. It is noted that equivalents fur the
particular embodiments
discussed in this description may practice the invention as well.
While various embodiments of the present invention have been described above,
it should
be understood that they have been presented by way of example only, and not of
limitation.
Likewise, the various diagrams may depict an example architectural or other
configuration for
the invention, which is done to aid in understanding the features and
functionality that may be
included in the invention. The invention is not restricted to the illustrated
example architectures
or configurations, but the desired features may be implemented using a variety
of alternative
architectures and configurations. Indeed, it will be apparent to one of skill
in the art how
alternative functional, logical or physical partitioning and configurations
may be implemented to
implement the desired features of the present invention. Also, a multitude of
different
constituent module names other than those depicted herein may be applied to
the various
partitions. Additionally, with .regard to flow diagrams, operational
descriptions and method.
claims, the order in which the steps are presented herein shall not mandate
that various
embodiments be implemented to perform the recited functionality in the same
order unless the
context dictates otherwise.
Although the invention is described above in terms of various exemplary
embodiments
and implementations, it should be understood that the various features,
aspects and functionality
described in one or more of theindividual embodiments are not limited in their
applicability to
the particular embodiment with which they are described, but instead may be
applied, alone or in
various combinations, to one or more of the other embodiments of the
invention, whether or not
such embodiments are described and whether or not such features are presented
as being a part
of a described embodiment. Thus the breadth and scope of the present invention
should not be
limited by any of the above-described exemplary embodiments.
Terms and phrases used in this document, and variations thereof, unless
otherwise
.expressly stated, should be construed as open ended as opposed to limiting.
As examples of the
foregoing: the term "including" should be road as meaning "including, without
limitation" or the
like; the term "example" is used to provide exemplary instances of the item in
discussion, not an
exhaustive or limiting list thereof; the terms "a" or "an" should be read as
meaning "at least
one," "one or more or the like; and adjectives such as "conventional,"
"traditional," "normal,"
"standard," "known" and terms of similar meaning should not be construed as
limiting the item
described to a given time period or to an item available as of a given time,
but instead should be
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read to encompass conventionz11õ traditional, normal,. or standard
technologies that may. be
available or known now or at any time in the future. Likewise, .where this
document refers to
technologies that would be apparent or .known to one of ordinary skill in the
art, such
technologies encompass those apparent or known to the skilled artisan now or
at any time in the
future.
.A group of items linked with the conjunction "and" should not be read as
requiring that
each and every one of those items he present in the grouping, but rather
Should be read as
"and/or" unless expressly stated otherwise_ Similarly, a group of items linked
with the
conjunction "or" should not be read as requiring mutual exclusivity among that
group, but rather
should also be read as "andior" unless expressly stated otherwise.
Furthermore, although items,
elements or components of the invention may be described or claimed in the
singular, the plural
is contemplated to be within the scope thereof unless limitation to the
singular is explicitly
stated.
The presence of broadening words and phrases such as "one or more," at least,"
"but not
limited to" or other like phrases in some instances shall not be read to mean
that the narrower
case is intended or required .in instances where such broadening phrases may
be absent. The use
a the term "module" does not imply that the components or functionality
described or claimed
as part of the module are all configured in a common package. Indeed, any or
all of .the various.
components of a module, whether control logic or other components, may be
combined in a
single package or separately maintained and may further be distributed across
multiple locations.
Additionally, the various embodiments set forth herein are described in terms
of
exemplary block diagrams, flow charts and other illustrations. As will become
apparent to one
of ordinary skill in the art after reading this document, the illustrated
embodiments and their
various alternatives may be implemented without confinement to the illustrated
examples. For
example, block diagrams and their accompanying description should not be
construed as
mandating a particular architecture or configuration.
